The White House was put on lockdown on Saturday after a loud noise was heard on the South Lawn moments before President Barack Obama and his family were due to take off by helicopter, a White House pool report said on Saturday.

It said the noise prompted Secret Service agents to move the 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.

"Lockdown continues, although it doesn't appear as though there's a high level of consternation among Secret Service walking outside," the pool report added later.

No further details were immediately available.

Obama and his family were 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.