(Reuters) - A White House lockdown triggered by a loud bang just moments before President Barack Obama and his family were due to take off by helicopter from the South Lawn has been lifted, media reports said on Saturday.

The reports quoted a Secret Service spokesman as saying a food vendor cart had caught fire nearby.

The New York Times said that at around the same time as the fire, a bomb-sniffing dog had detected something on a nearby vehicle, although the two incidents did not appear related. The vehicle was being checked.

A tweet by WTOP local radio quoted the Secret Service as saying the fire on a nearby street corner was not considered a serious threat and that the security situation was now contained.

White House pool reports earlier said a loud bang had prompted Secret Service agents to move the waiting press back into the briefing room, lock the doors and put the building on lockdown.

Obama and his family were not yet on the South Lawn when the noise was heard.

They had been about to depart by helicopter to Andrews Air Force Base to board Air Force One and travel to Selma, Alabama, to commemorate the 50th anniversary of a civil rights march.

The pool reports said the president and his family later departed by motorcade instead of helicopter to Andrews Air Force Base.

(Writing by Sandra Maler; Editing by Gareth Jones)