Undercover agents foiled a plot by a Somali-born teenager to carry out a car bomb attack at a crowded Christmas tree-lighting ceremony in Portland, Oregon, prosecutors said yesterday.

The 'bomb' turned out to be a dummy supplied by the agents as part of a sting.

Mohamed Osman Mohamud was arrested at about 5.45pm local time on Friday, just after he dialled a mobile phone that he thought would blow up a van laden with explosives.

But federal agents and local police swooped to take him into custody.

Federal court documents showed the sting operation began in June after an undercover agent learned that Mohamud had been in contact with an "unindicted associate" in Pakistan's Northwest Frontier region.

Mohamud is a naturalised US citizen.