A knife attack on a man at a London metro station in which the assailant reportedly screamed "this is for Syria" is being investigated by police as a terrorist incident.

The attack prompted the government to urge Britons on Sunday not to be intimidated.

A pool of blood near the ticket barriers at Leytonstone undergound station, about 10 kilometers east of central London, could be seen on footage posted on Twitter that also showed the suspect confronting police on Saturday evening.