Four more people died as anti-American protests sparked by a privately-produced film mocking Islam spread across the Middle East and North Africa, targeting US embassies.
The fatalities occurred in the Yemeni capital Sanaa on Thursday as police fired live rounds and tear gas to try to disperse an angry crowd of hundreds of protesters trying to storm the US mission.
The US embassy in Cairo came under siege for a third day as protests spread to several countries and territories including Bangladesh, Iran, Iraq, Israel and the Gaza Strip, Jordan, Kuwait, Sudan and Tunisia.
The United States attempted to keep a lid on the protests by spelling out that the film that set off the violence was privately-made by a small group of individuals and nothing at all to with anyone official.
"Let me state very clearly -- and I hope it is obvious -- that the United States government had absolutely nothing to do with this video," declared US Secretary of State Hillary Clinton.
"To us, to me, personally, this video is disgusting and reprehensible. It appears to have a deeply cynical purpose, to denigrate a great religion and to provoke rage," Clinton said, urging an end to the violence.
Enraged demonstrators continued their actions unabated and Islamist groups in several countries called for greater protests after Friday prayers, a prospect the White House admitted it feared as it tightened security at diplomatic missions around the globe.
The protests came as US and Libyan officials probe an attack on the consulate in Benghazi that killed the ambassador and three other US officials on Tuesday, amid growing speculation it was the work of jihadist militants rather than just demonstrators.
Libya Prime Minister Mustafa Abu Shagur told AFP in an exclusive interview that a "big advance" had been made in the probe into the Benghazi attack.
"We have some names and some photographs," he said. "Arrests have been made and more are under way as we speak."
A security official in Yemen said that as well as those killed in the clashes, another four people had been wounded, eight of them seriously. The unrest lasted from morning until late evening, he added.
Troops deployed on the rooftops of buildings around the US mission in Sanaa and police used water cannon and fired warning shots to drive out protesters who had breached the perimeter wall.
Witnesses said they saw three vehicles being torched by a group of demonstrators that gained access to the compound through an unguarded security gate.
Yemeni President Abdrabuh Mansur Hadi apologised to US President Barack Obama for the acts of a "mob" and ordered an investigation.
Obama called Hadi to express concern about the safety of American personnel at the US embassy there and thanked him for his swift condemnation of the violence.
US State Department spokeswoman Victoria Nuland said all embassy staff in Yemen were safe and accounted for.
Violence again rocked the Egyptian capital Cairo, where police fired tear gas to disperse protests outside the embassy by stone- and bottle-throwing demonstrators.
A total of 224 people were injured, the Egyptian health ministry said.
The violence began on Tuesday night when protesters stormed the Cairo embassy compound, tearing down the Stars and Stripes and replacing it with a black Islamic flag.
Egypt's Islamist President Mohamed Morsi has warned against resorting to violence but his angry statements about the film and failure to immediately apologize over the storming of the US embassy have Washington on edge.
Obama acknowledged late Wednesday that ties with the new Egypt were a "work in progress" and seemed to be signal a review of its status, by saying Cairo could neither be considered an ally nor an enemy.
More protests are expected Friday after Egypt's powerful Muslim Brotherhood, the Islamist movement through which Morsi rose to power, called for nationwide action following evening prayers.
And in Jordan, Salafist jihadists have said they plan to demonstrate after Friday midday prayers outside the US embassy in Amman.
Amid the mounting protests, Russian President Vladimir Putin said the Middle East was at risk of descending into "chaos."
"We are afraid that the region may descend into chaos, which is essentially what is happening already," he said.
Putin urged the new governments that rose to power in the Arab Spring uprisings to accept greater responsibility for security.
More details emerged about the four US nationals killed in the Benghazi consulate attack, which happened on the anniversary of the September 11 attacks on the United States.
One of those killed was a former Navy SEAL in Libya to help track down shoulder-launched surface-to-air missiles still unaccounted for following the downfall of former dictator Moamer Kadhafi, ABC News reported.
The Middle East crisis continued to reverberate in the US presidential race as Republican challenger Mitt Romney insisted that US power was vital in the region but halted his criticism of Obama's handling of the crisis.
The White House hopeful sparked a furore on Tuesday when he offered a quick and blunt rebuke of steps the Obama administration took to try and tamp down the rapidly escalating protests.
The catalyst for the bloody conflagration in the Muslim world was a privately-made film denigrating the Prophet Mohammed linked to evangelical and Coptic Christians in the United States.
The suspected producer is Nakoula Basseley Nakoula, a 55-year-old Copt living in California. It was promoted on the websites of two other Americans, extremist Christian pastor Terry Jones and another Copt, Washington-based lawyer Morris Sadek.
Both the State Department and the White House say there is nothing they can do to stop individuals producing inflammatory material because of freedom of speech laws enshrined in the First Amendment of the US constitution.