Tripoli — The U.S. ambassador to Libya and three other embassy staff were killed in a rocket attack on their car, a Libyan official said, as they were rushed from a consular building stormed by militants denouncing a U.S.-made film insulting the Prophet Mohammad.

Gunmen had attacked and burned the U.S. consulate in the eastern city of Benghazi, a centre of last year's uprising against Muammar Gaddafi, late on Tuesday evening. The Libyan official said the ambassador, J. Christopher Stevens, was being driven from the consulate building to a safer location when gunmen opened fire.

"I do condemn the cowardly act of attacking the US consulate and the killing of Mr. Stevens and the other diplomats," Deputy Prime Minister Mustafa Abushagur said.

Libyan officials spent the night in a manhunt trying to find those responsible for the killing of the American diplomat and three other American employees of the consulate. It was unclear whether the bodies of the Americans had been recovered.

The circumstances surrounding the death of Ambassador Stevens weren't immediately clear. The American official, who is usually based in the capital Tripoli, apparently was visiting Benghazi ahead of the planned opening of an American cultural centre there, according to a Libyan official.

The U.S. Embassy declined to comment on the news, with a diplomat in Tripoli saying that a statement would be released soon. A second Libyan official said that the newly elected General National Congress was expected to call a state of emergency in Libya later in the day.