Libya embassy violence was a 'planned terrorist attack'

US OFFICIALS are investigating the four-hour assault that killed the ambassador to Libya and three other Americans as a well-orchestrated terrorist attack, not a spontaneous reaction to an inflammatory movie about the Muslim prophet Muhammad.

President Barack Obama, who huddled with US State Department advisers and ordered flags to be lowered to half-mast, called the killing of American diplomats outrageous and vowed to track down the assailants.

''Make no mistake, justice will be done,'' Mr Obama said, adding that ''we must all unequivocally oppose the kind of senseless violence that took the lives of these public servants.''

Christopher Stevens was the first American ambassador to die in the line of duty in more than  three decades.

In about 15 minutes the assailants had breached the perimeter of the US embassy in Benghazi guarded by Libyan security forces and began setting fire to buildings, a US official said.

It took Libyan security forces and US personnel, repelled by flames, dark smoke and small-arms fire, four hours to regain control, the official said.

In response to the attacks, all but critical US diplomatic workers were pulled out of Libya and Mr Obama dispatched additional security personnel to the region, including about 50 marines to guard embassy workers in Libya's capital, Tripoli. Two US warships were reportedly moving towards the Libyan coast.

Mr Obama called the leaders of Egypt and Libya to discuss security co-operation following the violence. The Libya assault - and an earlier violent protest at the US embassy in Cairo - became a flashpoint in the presidential campaign.

The protest in Cairo was sparked by the film that denigrates Islam and Muhammad. Previous unflattering depictions of the prophet, particularly in cartoons in Denmark in 2005, had also ignited anger in the Muslim world.

The film was initially blamed for both the Cairo protest and the attack in Libya. But Senator John Kerry, the chairman of the foreign relations committee who was briefed by administration officials, said that the Libya attack appeared to be premeditated.

''The evidence was that [the Libya] attack was very well planned and very well structured,'' he said. ''This was not a mob.''

But exactly who it was, or whether the attack was linked to the anniversary of the September 11 attacks, remained unknown.

''Many details of what happened in Benghazi are still unknown or unclear,'' an administration official said, adding it was ''clearly a complex attack''.

Mr Stevens, 52, America's top diplomat in Libya since May, was trapped with two others inside the main consulate building, the official said.

Some specialists questioned whether the consulate's security was sufficient given the unrest in the country and growing reports of the presence of al-Qaeda-inspired militants.

US and Libyan officials took pains to stress that the deaths would not damage growing ties between the nations.

Last night, the US embassy in Sana'a, Yemen, was stormed by protesters, and smoke and gunfire were reported. And at least 13 people were injured in clashes outside the US embassy between Egyptian security forces and demonstrators angered by the video