The US ambassador to Libya was killed after being attacked by militiamen who stormed the American consulate in the eastern city of Benghazi, reports said.
Christopher Stevens was said to be among four US officials killed in the late Tuesday protest over a US-produced film that is said to have insulted the Prophet Muhammad.
The State Department has confirmed the death of an official but it has not named him.
The ambassador, who was on a short trip to Benghazi, apparently died of suffocation as a result of smoke inhalation after the mob set fire at the scene.
Libya's Supreme Security Committee spokesman Abdel-Monem Al-Hurr said, "One American staff member has died and a number have been injured in the clashes."
US Secretary of State Hillary Clinton confirmed "a staff member" died.
In Cairo, several thousands of Egyptians protested in front of the US embassy against the same movie, allegedly sponsored by Americans but not by the US government or Hollywood companies.