Washington - US Secretary of State Hillary Clinton confirmed late on Tuesday that a State Department official had been killed in an attack on America's consulate in the Libyan city of Benghazi.
"We are heartbroken by this terrible loss," Clinton said in a statement, adding that Washington was working with countries around the world to protect its missions after twin attacks in Libya and the Egyptian capital of Cairo.
"Our thoughts and prayers are with his family and those who have suffered in this attack," Clinton added in her statement.
The attack in Benghazi came after protesters earlier in the day stormed the US embassy in Cairo, scaling the walls and tearing down the US flag, on the sombre 11th anniversary of the September 11 attacks on the United States.
Clinton did not name the man who was killed when an armed mob protesting against a film deemed to be offensive to Islam attacked the consulate in Libya, using rocket-propelled grenades and eventually setting the mission on fire.
"Some have sought to justify this vicious behaviour as a response to inflammatory material posted on the internet," Clinton said in her statement.
"The United States 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."
Clinton said she had spoken with Libyan leader Mohamed al-Megaryef to co-ordinate extra support to help protect Americans working in Libya, and he had pledged his full co-operation.
Wave of violence
"In light of the events of today, the United States government is working with partner countries around the world to protect our personnel, our missions, and American citizens worldwide," Clinton said.
Benghazi, cradle of the revolution that ousted strongman Muammar Gaddafi last year, has seen a wave of violence in recent months, including attacks on Western targets, bombings of military buildings and the killings of army and security officers.
The United States helped build an international coalition which imposed a "no-fly" zone over Libya last year, helping the rebels to oust Gaddafi.
Washington has come under increasing pressure to increase its involvement in a similar rebellion in Syria, where the opposition is fighting to topple President Bashar Assad. But Tuesday's events in Libya could harden opposition to any full-scale US intervention in Syria.
The film at the centre of anti-US protests was made by an Israeli-American who describes Islam as a "cancer", the Wall Street Journal reported.
The movie, "Innocence of Muslims", was directed and produced by Sam Bacile, a 52-year-old real-estate developer from southern California.
"Islam is a cancer," Bacile told the newspaper of his crudely-produced film, which depicts the Prophet Mohammed variously sleeping with women, talking about killing children and referring to a donkey as "the first Muslim animal".
The film is being promoted by controversial Florida pastor Terry Jones, who has drawn protests in the past for burning the Koran and vehemently opposing the construction of a mosque near Ground Zero in New York.