The US ambassador to Libya and three other embassy staff were killed in Tuesday’s attack on the US consulate in Benghazi, say reports.
It is not clear if the ambassador was killed in his car or the compound.
“The US ambassador and three staff members were killed when gunmen fired rockets at them,” an official in Benghazi told Reuters.
Gunmen attacked the compound on Tuesday evening, clashing with Libyan security forces before the latter withdrew as they came under heavy fire.
Reporters on the scene said they could see looters raiding the building, walking off with desks, chairs and washing machines.
It followed an attack in the Egyptian capital Cairo, when protesters climbed the walls of the US embassy and tore down the American flag and burned it.
The protests were sparked by outrage over a video being promoted on YouTube by extreme anti-Muslim groups in the US.
Demonstrators say the \$5m film, called The Innocence Of Muslims, insults the Prophet Mohammed.
It apparently depicts him as a fraud, showing him having sex and calling for massacres.
Many Muslims consider any depiction of the prophet to be offensive.
Earlier, Secretary of State Hillary Clinton said she had called the Libyan government to coordinate more protection for Americans in the country.
“Some have sought to justify this vicious behaviour as a response to inflammatory material posted on the internet,” she said.
“The US deplores any intentional effort to denigrate the religious beliefs of others. Our commitment to religious tolerance goes back to the very beginning of our nation.
“But let me be clear: There is never any justification for violent acts of this kind.”
Libyan officials said it remained unclear whether the two protests in Benghazi and Cairo had been coordinated.
Benghazi was the launch-pad of last year’s revolution which overthrew Colonel Muammar Gaddafi and is a seat of Islamist politics in Libya.