The United States is investigating whether the deadly attack on a US consulate in Libya was a co-ordinated terrorist strike to mark the anniversary of the 9/11 atrocity.
Ambassador Chris Stevens and three other American officials died as gunmen fired rocket-propelled grenades and set fire to the US compound in Benghazi.
It had been thought it was a spontaneous protest provoked by an American-made anti-Islam film, which is being promoted on YouTube and is said to insult the Prophet Mohammed.
But a US counterterrorism official has said the Benghazi violence was "too co-ordinated or professional" to be spontaneous.
The Pentagon has announced it is moving two warships to the Libyan coast and a group of Marines has been dispatched to the Libyan capital, Tripoli.
The ships, which carry Tomahawk missiles, do not have a specific mission but will give commanders flexibility to respond to any mission ordered by President Barack Obama, officials said.
Mr Stevens, 52, died of "severe asphyxia", apparently from smoke inhalation, according to a Libyan doctor who said he tried for 90 minutes to revive him.
Libyan security forces had withdrawn from the compound as they came under heavy fire. Reporters at the scene said they could see looters raiding the building, walking off with desks, chairs and washing machines.
President Obama condemned the "outrageous and shocking" attack. "Make no mistake - justice will be done," he said.
The violence followed an attack in Cairo, where protesters climbed the walls of the US embassy, tore down an American flag and burned it.
Protests in the Egyptian capital continued overnight as demonstrators clashed with police near the US embassy. Police were pelted with rocks and responded with a charge at the protesters and continued with their firing tear gas to disperse the crowd.
In Tunisia, police have fired tear gas and rubber bullets into the air to disperse a protest by around 200 people near the US embassy in the capital Tunis, apparently about the film.
Opponents say the \$5m (£3.1) film, called The Innocence Of Muslims, depicts Mohammed as a fraud and shows him having sex and calling for massacres.
The US Navy has already moved the destroyer, the USS Laboon, to a position off the Libyan coast and the USS McFaul is en route and should be stationed off the coast within days.
Pentagon spokesman George Little said: "Without commenting on specific ship movements, the United States military regularly takes precautionary steps when potential contingencies might arise in a given situation. That's not only logical in certain circumstances, it's the prudent thing to do."