The U.S. ambassador to Libya and three other Americans were killed after protesters angry over a film that ridiculed Islam's Prophet Muhammad stormed the U.S. consulate in the eastern Libyan city of Benghazi.
"I strongly condemn the outrageous attack on our diplomatic facility in Benghazi, which took the lives of four Americans, including Ambassador Chris Stevens," President Barack Obama said Wednesday in a statement. "Right now, the American people have the families of those we lost in our thoughts and prayers."
The statement said Foreign Service Information Management Officer Sean Smith, a father of two, was also killed.
Stevens was the first U.S. ambassador killed during an assignment since Adolph Dubs was slain in an exchange of gunfire during a kidnapping attempt in Kabul, Afghanistan, in 1979.
Earlier, three Libyan officials told The Associated Press that Stevens was killed Tuesday night when he and a group of embassy employees went to the consulate to try to evacuate staff.
State requests more Marines
U.S. officials told NBC News that the State Department had requested additional units of Marines be airlifted to Libya to bolster security at the embassy there.
The Air Force was also preparing to send two aircraft to Libya to provide medevac flights for any wounded or injured if necessary.
The officials said that as of now there was no plan for an all-out evacuation of U.S. government personnel from Libya.
On Tuesday in Benghazi, a large mob stormed the U.S. consulate, with gunmen firing their weapons, said Wanis al-Sharef, an Interior Ministry official in Benghazi.
A witness said attackers fired automatic weapons and rocket-propelled grenades at the consulate as they clashed with Libyans hired to guard the facility.
Outnumbered by the crowd, the Libyan security forces did little to stop them, al-Sharef said.
'A terrifying day'
The crowd overwhelmed the facility and set fire to it, burning most of it and looting the contents, witnesses said.
"I heard nearly 10 explosions and all kinds of weapons. It was a terrifying day," a witness who refused to give his name because he feared retribution told the AP.
The circumstances surrounding the death of Stevens and the other Americans were not immediately clear.
"I have directed my Administration to provide all necessary resources to support the security of our personnel in Libya, and to increase security at our diplomatic posts around the globe," Obama's statement said.
"While the United States rejects efforts to denigrate the religious beliefs of others, we must all unequivocally oppose the kind of senseless violence that took the lives of these public servants," he said.
Stevens was typically based in the Libyan capital, Tripoli, but was apparently visiting Benghazi for the opening of an American cultural center there, The Wall Street Journal said.
According to a biography posted on the State Department's website, Stevens was a career Foreign Service officer. He had twice served in Libya -- from 2007 to 2009 and in 2011 -- before being named ambassador in May.
Stevens, who was born and raised in northern California, had also held overseas posts in Israel, Saudi Arabia and Syria.
Obama called Stevens "a courageous and exemplary representative of the United States."
Protests in Cairo
Demonstrations also broke out Tuesday in Egypt, where protesters scaled the walls of the U.S. Embassy in Cairo, and tore and replaced the American flag with an Islamic banner. Demonstrations continued outside the American facility Wednesday afternoon.
Tuesday's attacks were the first such assaults on U.S. diplomatic facilities in either country, at a time when both Libya and Egypt are struggling to overcome the turmoil following the ouster of their longtime leaders, Moammar Gadhafi and Hosni Mubarak, in uprisings last year.
The protests in both countries were sparked by outrage over a film ridiculing Muhammad. It was produced by an Israeli filmmaker living in California and was being promoted by an extreme anti-Muslim, Egyptian Christian campaigner in the United States. Excerpts from the film dubbed into Arabic were posted on YouTube.
The 14-minute trailer of the movie, posted on the website in an original English version and another dubbed into Egyptian Arabic, depicts Muhammad as a fraud, a womanizer and a madman in an overtly ridiculing way, showing him having sex and calling for massacres.
YouTube's guidelines call for removing videos that include a threat of violence, but not those that only express opinions. YouTube's practice is not to comment on specific videos.
Intended to be provocative
Sam Bacile, a 56-year-old California real estate developer who identifies himself as an Israeli Jew and who said he produced, directed and wrote the two-hour film, called "Innocence of Muslims," said he had not anticipated such a furious reaction.
Speaking by phone to the AP from an undisclosed location, Bacile, who went into hiding Tuesday, remained defiant, saying Islam is "a cancer" and that he intended his film to be a provocative political statement condemning the religion.
Bacile said the film was produced in English and he does not know who dubbed it in Arabic. The full film has been shown once, to a mostly empty theater in Hollywood earlier this year, he said.
Morris Sadek, an Egyptian-born Christian in the United States known for his anti-Islam views, told the AP from Washington that he was promoting the video on his website and on certain TV stations, which he did not identify.
For several days, Egyptian media have been reporting on the video, playing some excerpts from it and blaming Sadek for it, with ultraconservative clerics going on air to denounce it.