WASHINGTON (Reuters) - White House staff and reporters returned to the building after being briefly evacuated on Friday night because of an intruder on the grounds, a Reuters witness said.

United States President Barack Obama had just left aboard the Marine One helicopter for Camp David, the presidential retreat in nearby Maryland, when armed, uniformed Secret Service personnel ordered the press corps and members of the White House staff to evacuate.

They were allowed back in after about half an hour, the witness said.

A Secret Service officer said the alert was sparked by an intruder who jumped over the White House fence.

No further details were immediately available.