Gunmen and security forces clashed at the US consulate office in the eastern Libyan city of Benghazi as the armed group protested over a film being produced in the United States.
The incident followed a protest in neighbouring Egypt where demonstrators scaled the walls of the US embassy, tore down the American flag and burned it during a protest over what they said was a film that insulted Prophet Mohammed.
A US embassy source said there had been "an attack" on the diplomatic office in Benghazi, but gave no further details.
Benghazi, the cradle of last year's uprising against Muammar Gaddafi, has been hit by several bombings and attacks on international convoys and organisations as well as some Western missions.
In June, an explosive device was dropped from a passing car outside the offices of the US diplomatic mission.
The blast that followed slightly damaged the gate in front of the building.
A week later, a British embassy convoy was hit about 300 metres from the British consulate office in the city.