The White House was temporarily placed on lockdown after a loud noise was heard moments before the President and his family were due to leave the building.

It came moments before Barack Obama and his family were due to depart by helicopter to Andrews Air Force Base to board Air Force One and travel to Selma, Alabama, to mark the anniversary of the civil rights marches there.

The noise prompted the Secret Service to move members of the press back into the briefing room, lock the doors and put the building on lockdown.

This was lifted around 30 minutes later.

The loud noise is believed to have been caused by a vendor cart catching fire near the White House.

Secret Service spokesman Brian Leary said this happened at about 10am at 15th and G streets.

Firefighters responded and contained the blaze, he said.

DC Fire and EMS said one patient was checked over but was not taken to hospital.

At around the same time, a bomb-sniffing dog detected something on a vehicle at 16th Street and Constitution Avenue, Mr Leary added.

This vehicle is being examined by the Secret Service and other law enforcement officials.

There is no indication that the two events are related.

White House pool reports said the president and his family later left by motorcade for Andrews Air Force Base.

Originally posted at 16.15