US Secretary of State Rex Tillerson condemned Wednesday's attack outside parliament in London and expressed his condolences to the victims and their families.

"We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference," he said in a statement.

An assailant stabbed a policeman and was shot by an officer in what police describe as a "terrorist incident", which also involved a car crashing into pedestrians.