The United States embassy compound in the Libyan capital Tripoli has been raided by militiamen, but no Americans were present as the embassy was evacuated more than a month ago.
The attack on Sunday comes as fighting between rival militias rages in Tripoli and Benghazi, the scond city in the east.
Even though the US suspended its mission in Tripoli for the second time in three years in July, the US Secretary of State John Kerry had said that the embassy was not closed, and its staff had been evacuated to neighbouring Tunisia.
The US ambassador to Libya said the embassy was being safeguarded and had not been ransacked.
Deborah Jones said on Twitter that there was no indication the complex, which has been mothballed since staff were evacuated, had been damaged.
"To my knowledge & per recent photos the US Embassy Tripoli chancery & compound is now being safeguarded and has not been ransacked," she said.
A video that emerged on YouTube showed a crowd of cheering men standing on buildings near a swimming pool.
Several men are seen diving into the pool off a nearby rooftop, to a chorus of cheers.
Jones, who is in Malta, stressed however that the area depicted in the video "appears to be a residential annex" of the US mission, adding that she "cannot say definitively since [I am] not there."
"Those who have actually visited the embassy know the truth," she posted.
An AFP photographer who was at the embassy compound on Sunday said Islamist militiamen had moved in to buildings in the complex.
Members of the Fajr Libya (Libya Dawn) group said they had gone in to secure the complex of several villas in southern Tripoli to prevent it from being looted.
In February 2011, the embassy suspended operations amid the uprising that eventually toppled longtime leader Muammar Gaddafi. After the formation of a transitional government in July 2011, the embassy reopened in September.
The Obama administration has been particularly sensitive about security of US government employees in Libya since the September 11, 2012, attack on the US mission in Benghazi that killed ambassador Chris Stevens and three other Americans.
Tripoli has been embroiled for weeks in inter-militia violence that has killed and wounded dozens on all sides.