President Barack Obama has vowed to bring to justice those who carried out the attack that killed the American ambassador to Libya.
However, Mr Obama said the attack on the consulate in the eastern city of Benghazi on Tuesday would not harm ties between the United States and the new Libyan government, the BBC reports.
Ambassador J Christopher Stevens, 52, died after gunmen stormed the consulate amid protests over an anti-Islamic film produced in the US.
Mr Obama said on Wednesday that the killings will not hurt US-Libyan ties, but added: "Make no mistake - justice will be done."
The US has sent destroyers and 50 Marines to Libya to secure the main American embassy in the capital, Tripoli, where staff numbers are being cut to emergency levels. It has also begun evacuating all its staff from its mission in Benghazi.
The BBC reports many people in Libya are still armed following the conflict that overthrew Colonel Muammar Gaddafi last year. Analysts say the attack raises serious concerns about stability there and the ability of the new administration to maintain security.
US officials said the consulate compound in Libya began taking heavy fire about 10pm (local time) on Tuesday and the main building was in flames soon after.
Three other Americans were also killed, including Sean Smith, a State Department employee, in what the White House described as a "complex" attack.
Libyan and US security forces tried to retake the compound several times, US officials said, but only succeeded early on Wednesday. Mr Smith was found dead inside the compound and a Libyan doctor who treated him said he died of severe asphyxiation.
US officials said Washington was investigating whether the attack was organised in advance, rather than a spontaneous assault sparked by demonstrations over the film.
Officials told Reuters there were suspicions that a militia known as the Ansar al-Sharia brigade was involved in the attack. The group has denied the claim. They also cited reports suggesting that al-Qaeda's north Africa-based affiliate, known as al-Qaeda in the Islamic Maghreb, may have been involved.
Meanwhile, protesters in Egypt have clashed with security forces after crowds returned to the US embassy in Cairo for a second evening of demonstrations over the film Innocence of Muslims. Riot police fired tear gas at protesters, some of whom were seen carrying petrol bombs, according to witnesses.