BBC-The US ambassador to Libya is among four Americans killed in an attack on the US consulate in Benghazi, President Barack Obama has confirmed.
Unidentified armed men stormed the grounds overnight amid uproar among Muslims over a US-produced film said to insult the Prophet Muhammad.
They shot at buildings and threw handmade bombs into the compound.
It has not been confirmed how the ambassador, J Christopher Stevens, and the others died.
Protesters against the film also attacked the US embassy in Cairo on Tuesday night.
Speaking in the Rose Garden at the White House, President Obama told reporters the attack would not break the bonds between the US and the new Libyan government.
But he added: “Make no mistake – justice will be done.”
The US is deploying a marine anti-terrorism team to Libya to bolster security after the attack, a US defence source told reporters in Washington.
Libya’s interim leader, Mohammed Magarief, apologised to the US over the killings, which he called “cowardly criminal acts”.
Reports say a militia known as the Ansar al-Sharia brigade was involved in the attack, but the group has denied the claim, the BBC’s Rana Jawad in Tripoli says.
Reports suggest that Ambassador Stevens and his staff went to the consulate in an attempt to evacuate the site after it was attacked.
They were reportedly trying to leave the consulate building for a safer location when gunmen launched an intense attack, apparently forcing back security guards.
“The American ambassador and three staff members were killed when gunmen fired rockets in their direction,” a Libyan official in Benghazi told Reuters news agency.
The Libyan doctor who treated Mr Stevens in hospital said he had died of severe asphyxiation, apparently from smoke inhalation, with no other injuries, and that he had tried for 90 minutes to revive him.
He was the only American brought into the Benghazi Medical Centre and initially nobody realised he was the ambassador, Ziad Abu Zeid told the Associated Press news agency.
A second US man killed in the attack was named as Sean Smith, a father of two who was employed as an information management officer.