US Secretary of State Hillary Clinton confirmed late 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 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.
She 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, using rocket-propelled grenades and eventually setting the mission on fire.
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, as Americans marked the 11th anniversary of the 9/11 terror attacks on the United States.
"Some have sought to justify this vicious behavior 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 coordinate extra support to help protect Americans working in Libya, and he had pledged his full cooperation.
"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, a stronghold of Islamist extremists and cradle of the revolution that saw strongman Libyan Moamer Kadhafi captured and killed 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.