The US State Department said on Wednesday it was closely monitoring the "concerning situation" near to Britain's Houses of Parliament and offered to assist in any way after a policeman was stabbed and several people injured in an attack.

State Department spokesman Mark Toner urged Americans in London to contact their families to let them know they are safe.

"We stand ready to provide all possible consular assistance should we become aware of any affected U.S. citizens," he said in a statement.