The victim, 23, was walking along Ring Road, about 10.30pm when one youth approached him asking for a cigarette.

As the student went to give the youth the cigarette, he was surrounded by four other youths and a knife was produced.

One of them, estimated to be aged as young as 12, punched the victim in the face with such force the iPod he was wearing fell from his ears.

The student ran from the scene to a nearby library and called for help.

Police said the iPod had mostly Korean music on it and the cigarettes, which the victim also dropped, are of South Korean brand This Plus.

The attacker is described as being aged 12 or 13, about 165 centimetres tall, of thin build and had short blond hair.

He was wearing a light coloured t-shirt and blue knee-length pants.

A second robber was described as being aged 15 or 16, about 175 centimetres tall, of thin to medium build with short hair.

The others are believed to be aged between 13 and 16.