Malaysian officials have confirmed an oil slick has been sighted in Vietnamese waters and said Vietnamese ships were at the site investigating whether it was from the plane.
Investigators checking the identities of all the 227 passengers on the flight, particularly those who bought tickets through China Southern Airlines, which is a code share partner with Malaysia Airlines.
As well as the two passengers who appear to have been travelling on stolen passports, suspicions have been raised about two other passengers, Reuters quoted a security official in Kuala Lumpur as saying.
Malaysian media reported they were recorded in the flight manifest as European passengers.
Six nations are now desperately searching for the aircraft. Amid the questions about the weather, mechanical failure or human error, the discovery that two passengers were flying on stolen passports has sparked fears that foul play may well be the answer.
Italian Luigi Maraldi, 37, was on holiday in Thailand and immediately phoned home after seeing on the news that an Italian with his name was on the vanished airliner - and before his father had seen the news.
His passport, as well as that of Austrian national Christian Kozel, had been stolen while in Thailand.
Malaysian authorities are investigating the identities of at least another two passengers on the flight, in addition to two passengers who boarded using stolen passports, Reuters reported. The agency stated that authorities are checking the identities of the passengers who had brought their tickets through China Southern Airlines.
Malaysian aviation authorities said Sunday morning there has been no confirmed sighting of the plane carrying 239 people, including six Australians.
“We have not been able to locate anything. We have not been able to see anything,” said Azhaddin Abdul Rahman, deputy chief of aviation.
There were no reports of distress calls, emergency-beacon signals or bad weather and no indications why a plane would lose touch in one of the safest phases of flight.Vietnamese air force planes have spotted two large oil slicks off the southern tip of Vietnam which may be from the missing Malaysian jetliner that was carrying 239 people, including six Australians.
‘‘I think the two oil slicks are very likely linked to the missing plane,’’ said Vice-Admiral Ngo Van Phat, who is helping to direct the search mission.
‘‘However, we have to check carefully once our rescue boats get access to the area.’’
While there was no information pointing to a possible bomb or terror attack, Malaysia is studying all possibilities, Acting Transport Minister Hishammuddin Hussein told reporters.
The stolen Austrian passport belonged to 30-year-old Christian Kozel, who reported the theft in 2012 in Phuket, Thailand. He was contacted and found to be ‘‘well,’’ said Martin Weiss, a foreign ministry spokesman said.
Mr Maraldi, an Italian national also shown on the manifest, had reported his passport stolen last August, according to Italian media.
Malaysia Airlines, meanwhile, has advised immediate family members of those on the flight to gather at Kuala Lumpur International Airport; the airline stated it would pay for travel arrangements and expenses.
Mumbling and static were heard from the cockpit of missing Malaysia Airlines flight 370 before connection was lost, according to a pilot who was flying another plane in the area.
“There was a lot of interference … static … but I heard mumbling from the other end,” said the pilot who was asked to make contact with flight 370 by Vietnamese air traffic control.
“That was the last time we heard from them, as we lost connection,” he said.
The pilot, who asked not to be named, told Malaysia’s New Sunday Times newspaper said he established contact with flight 370 as he was flying a Boeing 777 to Narita, Japan.
His plane was 30 minutes ahead of the missing plane.
The pilot said he thought nothing of losing contact, as it was not an unusual, until it was confirmed that flight 370 never landed.
“If the plane was in trouble, we would have heard the pilot making the mayday distress call. But I am sure that, like me, no one else up there heard it,” he said.
“Following the silence, a repeat request was made by the Vietnamese authorities to retry establishing contact with them.”
The pilot said the voice he heard may have been either Captain Zaharie Ahmad Shah, 53, or co-pilot Fariq Abdul Hamid, 27.
“But I am sure it was the co-pilot,” he said.
A Queensland couple who wanted to spend more time travelling in their older age were two of the six Australian passengers on board the missing Malaysia Airlines flight.
The Boeing 777 aircraft, carrying 239 people, vanished while flying across the ocean between Malaysia and Vietnam early Saturday morning.
Neighbours said Catherine and Robert Lawton, aged 53 and 57, had already been on a trip to Asia and were "looking to see a bit of the world" now their three daughters had moved out.
Springfield Lakes resident Robbie Daintith, who lives across the road from the couple and would often put their bins out when they travelled, said they were "lovely people" who adored their young grandchildren.
"We knew from talking to them that they were planning to go away for a period of time again. They did a similar trip this time last year," Mr Daintith said.
"They were lovely people who always said hello, were always happy to have a chat and always offering to help out in any way they could."
He said Ms Lawton didn't work because of a visual impairment but her husband was employed.
The pair is understood to have been travelling on the flight with Brisbane couple Rodney and Mary Burrows.
Mr Burrows was a long-time employee with Energex, one of the largest energy companies in Queensland. It is believed he took a redundancy package two years ago.
Two other Australians on board the flight have been identified as NSW couple Yuan Li, 32, and Naijun Gu, 31.
Yuan LI and Naijun Gu are listed as company director and secretary of Y & J Australia PTY Ltd, respectively.
It believed Li was born in Beijing and Gu in Shanghai.
However, while their address is listed for a terrace in a residential complex in Koorooma Place, Sylvania, little else is known about them.
Neighbours said the couple bought the terrace in 2009 but were unsure if they actually lived there or simply owned the property and were renting it out to other people.
The terrace was put up for auction and sold just before Christmas.
"They might have owned the place but I doubt they ever lived there,” a neighbour said. “They were a bit reclusive.”
Paul Weeks, who had been living in Perth, is one of two missing New Zealander passengers. He was en route to a new job in Mongolia.
New Zealand media is reporting the father of two left Christchurch in search of a better life and for his family after the city's devastating earthquakes.
The Department of Foreign Affairs and Trade has released a statement saying it "fears the worst" for the Australian passengers.
Department of Foreign Affairs and Trade spokeswoman Kerin Ayyalaraju said Australian consular officials were in "urgent and ongoing contact" with Malaysia Airlines, and were speaking with distraught family members in Australia and offering "all possible consular assistance".
The plane vanished somewhere off Vietnam's Tho Chu Island, northwest off the country's southernmost Cape Ca Mau. The airline lost contact with the aircraft after it departed Kuala Lumpur.
It was expected to land in Beijing at 6.30am carrying 239 people, including two infants and 12 crew members.
Passengers included 152 Chinese people, 38 Malaysians, 12 Indonesians, three Americans and two Canadians. There were also passengers from New Zealand, Italy, France, Ukraine, Russia, Netherlands, Austria and Taiwan.
If all passengers are found dead, it will be the world's worst air tragedy in a decade.
A 20-kilometre long oil slick spotted between Malaysia and Vietnam was thought to be the first sign that the plane went down in the waters between southernmost Vietnam and northern Malaysia, according to Vietnam's director of civil aviation.
"An AN26 aircraft of the Vietnam Navy has discovered an oil slick about 20 kilometers in the search area, which is suspected of being a crashed Boeing aircraft - we have announced that information to Singapore and Malaysia and we continue the search," Lai Xuan Thanh, the director of the Civil Aviation Administration of Vietnam, told the New York Times in reporting the sighting of the slick.
Mr Thanh said the oil on the surface on the water was somewhat closer to Vietnam than Malaysia at the mouth of the shallow Gulf of Thailand.
The search efforts
The missing plane has sparked a multinational response.
Malaysia's prime minister, Najib Razak, said in a statement that 15 aircraft and nine ships were searching for the missing plane. Without saying where his government suspected that the plane might have disappeared, he added, "Our priority now is to widen the search area and provide support to relatives of those missing."
The airline released its seventh press release about the on-going mystery early Sunday afternoon, stating that its search and rescue teams have been unable to detect the whereabouts of the aircraft.
‘‘The airline is doing its utmost to provide support to the affected family members, this includes immediate financial aid,’’ the statement said.
The airline has deployed a team of 94 ‘caregivers’ to assist the family of missing passengers and crew; more support staff will be sent to Beijing, where the plane was to land, later today.
A command centre will be established in either Kota Bharu, Malaysia or Ho Chi Minh, Vietnam, as soon as the aircraft is located. ‘‘The airline is continuously working with the authorities in providing assistance. In fearing for the worst, a disaster recovery management specialist from Atlanta, USA will be assisting Malaysia Airlines in this crucial time.’’
The United States Seventh Fleet said in a Twitter message that it was sending a destroyer, the USS Pinckney, and a P-3C maritime surveillance aircraft to join the search for Flight MH370.
The US’s Federal Bureau of Investigation will join the international hunt for the aircraft and its passengers.
A Texas business, Freescale Semiconductor, has confirmed that 20 of its employees were on the flight.
China has also sent two maritime rescue ships to the South China Sea to help in any rescue, state television said on one of its microblogs.
The aircraft's pilot was Zaharie Ahmad Shah, a 53-year-old Malaysian captain, who has flown more than 18,000 hours. The first officer was Fariq Ab Hamid, 27, also of Malaysia, who has flown more than 2700 hours.
Malaysia Airlines initially reported seven Australians were on board, but the number was quickly revised to six.
On Saturday afternoon, the airline said it had been successful in contacting about 80 per cent of passengers' families.
Vietnam's official website said the plane disappeared in Vietnam's airspace.
"The plane lost contact in Ca Mau province airspace before it had entered contact with Ho Chi Minh City air traffic control," a statement posted on the website said.
Conflicting reports surfaced on Saturday afternoon over claims in Vietnamese and Chinese media that the missing plane's signal had been detected in the middle of the ocean.
Vienamese news site VN Express had quoted Pham The Hien, director of the country's search and rescue co-ordination centre, saying a signal was detected 230 kilometres south-west of Cape Ca Mau. But he later said those reports had been inaccurate, and his team was continuing to look for the missing plane's signal.
Distraught families in Beijing
With the arrivals board at Beijing's international airport still showing the Malaysian Airlines flight was delayed, distraught family members were being shepherded by police and airport staff to a nearby hotel to await further information.
One woman, Zhai Le, said her friend was on board the flight.
"They keep saying there's no information," she said through tears.
Another man, who declined to be named, said he had been waiting for his boss, a French national, when he heard the news.
An unconfirmed report from a flight tracking website said the plane had plunged more than 200 metres and changed course in the last minute that it had transmitted data.
Chinese authorities have reportedly said the flight never entered Chinese airspace.
"It doesn't sound very good," retired American Airlines captain Jim Tilmon told CNN on Saturday.
He said that the route was mostly overland, which meant that there would be plenty of radars and radios to contact the plane.
"I've been trying to come up with every scenario that I could just to explain this, but I haven't been very successful."
He said the plane was "about as sophisticated as any commercial airplane could possibly be".
Uncertainty over timing
Malaysia Airlines denied reports circulating on the internet the plane had landed safely in Nanjing China.
One uncertainty about the flight involved when it disappeared from radar and how quickly the search began in the Gulf of Thailand. Malaysia Airlines said that the plane took off at 12.41 am Malaysia time, and that the plane disappeared from air traffic control radar in Subang, a suburb of Kuala Lumpur, at 2.40 am.
That timeline seemed to suggest that the plane stayed in the air for two hours — long enough to fly not only across the Gulf of Thailand but also far north across Vietnam. But Mr Lindahl of Flightradar 24 said that the last radar contact had been at 1.19am, less than 40 minutes after the flight began.
A Malaysia Airlines spokesman said on Saturday evening that the last conversation between the flight crew and air traffic control in Malaysia had been around 1.30 am, but he reiterated that the plane had not disappeared from air traffic control systems in Subang until 2.40am.
Fuad Sharuji, from Malaysia Airlines' operation control centre, said the pilots made no distress call.
The missing plane is believed to have been involved in a crash in August, 2012, when it damaged the tail of a China Eastern Airlines plane at Shanghai Pudong Airport, according to unconfirmed reports.
In the incident, the tip of the wing of the Malaysia Airlines Boeing 777 broke off.
Malaysia Airlines said its "thoughts and prayers" were with all the passengers on board the missing plane, and their families.
"[The] focus of the airline is to work with the emergency responders and authorities and mobilise its full support," chief executive Ahmad Jauhari Yahya said.
Malaysia Airlines is the national carrier of Malaysia and one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
The airline said the public can call +60-378841234 for information about the plane.
There were no storms in the area of the South China Sea where the plane was flying across. The weather was generally fine with light clouds.
Malaysia Airlines' vice president of operations told CNN that no distress call or problems were reported from the aircraft prior to its disappearance. The plane was reportedly flying at 35,000 feet at the time.
"We are extremely worried," Chinese Foreign Minister Wang Yi told reporters in Beijing. "The news is very disturbing. We hope everyone on the plane is safe."
The plane had enough fuel to fly for seven hours, one hour more than the flight time to Beijing.
Boeing said in a statement that it was assembling a team of technical experts to advise the national authorities investigating the disappearance of the aircraft.