"The ambassador was killed along with three other officials," said Wanis al-Sharif, the deputy minister of the interior. Stevens' death in Tuesday's attack was confirmed in a tweet by Mustafa Abu Shagur, the deputy prime minister.
"The Libyan ambassador and three staff members were killed when gunmen fired rockets at them," the official i told Reuters from Benghazi, although it was unclear if the ambassador was in the consulate or his car at the time of the attack.
Earlier, protesters angered over a film that ridiculed Islam's Prophet Mohammed have fired gunshots and burned down the US consulate in the eastern Libyan city of Benghazi, killing one American, AFP reported.
In Egypt, protesters scaled the walls of the US embassy in Cairo, and tore and replaced the American flag with an Islamic banner.
Tuesday's attacks were the first such assaults on US 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, Muammar Gaddafi and Hosni Mubarak, in
uprisings last year.
The protests in both countries were sparked by outrage over a film ridiculing Mohammed produced by an Israeli filmmaker living in California and 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.
US Secretary of State Hillary Clinton confirmed that a State Department officer had been killed in the protest at the US consulate in Benghazi.
She strongly condemned the attack and said she had called Libyan President Mohammed el-Megarif "to coordinate additional support to protect Americans in Libya".
Clinton expressed concern that the protests might spread to other countries. She said the US is working with "partner countries around the world to protect our personnel, our missions, and American citizens worldwide".
In Benghazi, a large mob stormed the US 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, Libyan security forces did little to stop them, al-Sharef said.
The crowd overwhelmed the facility and set fire to it, burning most of it and looting the contents, witnesses said.
One American was shot to death and a second was wounded in the hand, al-Sharef said. He did not give further details.
The violence at the consulate lasted for about three hours, but the situation has now quieted down, said another witness.
Hours before the Benghazi attack, hundreds of mainly ultraconservative Islamist protesters in Egypt marched to the US embassy in downtown Cairo, gathering outside its walls and chanting against the movie and the US. Most of the embassy staff had left the compound earlier because of warnings of the upcoming demonstration.
"Say it, don't fear: their ambassador must leave," the crowd chanted.
Dozens of protesters then scaled the embassy walls, and several went into the courtyard and took down the American flag from a pole. They brought it back to the crowd outside, which tried to burn it, but failing that tore it apart.
