US president Barack Obama said today he had ordered his administration to do whatever is necessary to protect Americans abroad, as US diplomatic compounds in the Middle East faced continuing violent protests.
Mr Obama, speaking at a re-election campaign rally in Golden, Colorado, also said he and his aides had been in contact with other governments "to let them know they've got a responsibility to protect our citizens."
Mr Obama was speaking after the US ambassador to Libya and three other diplomats were killed after gunmen attacked the US consulate and a safe house refuge in Benghazi on Tuesday night.
The attackers were part of a mob blaming America for a film they said insulted the Prophet Muhammad.
Demonstrators attacked the US embassies in Yemen and Egypt today in protest at the film they consider blasphemous to Islam, and American warships headed to Libya after the US ambassador there died in related violence earlier this week.
Hundreds of Yemenis broke through the main gate of the heavily fortified compound in the capital Sanaa, shouting "We sacrifice ourselves for you, Messenger of God". They smashed windows of security offices outside the embassy and burned cars.
"We can see a fire inside the compound and security forces are firing in the air. The demonstrators are fleeing and then charging back," one witness said.
A security source said at least 15 people were wounded, some by gunfire, before the Yemeni government ringed the area with troops.
An embassy spokesman said its personnel were safe.
In Egypt, protesters hurled stones at a police cordon around the US embassy in central Cairo after climbing into the embassy compound and tearing down the American flag.
The state news agency said 13 people were hurt in violence which erupted late last night, following initial protests on Tuesday.
During a similar protest on Tuesday at the US consulate in Benghazi, Libyan Islamists staged military-style assaults on the mission and a safe house refuge.
It was the 11th anniversary of al-Qaeda's attacks on the United States on September 11th, 2001.
The US ambassador Christopher Stevens and three other Americans died in the assaults, carried out with guns, mortars and grenades. Eight Libyans were injured.
Mr Obama vowed to "bring to justice" those responsible for the attack, which US officials said may have been planned in advance. US secretary of state Hillary Clinton said Washington had nothing to do with the video, which she called "disgusting and reprehensible".
The US military moved two destroyers towards the Libyan coast, in what an official said was a move to give the administration flexibility for any future action against Libyan targets.
The US military also dispatched a Marine Corps anti-terrorist team to boost security in Libya, whose leader Muammar Gadafy was ousted in a US-backed uprising last year.
Mr Obama said security was being increased at US diplomatic stations around the globe.
Nigeria put police on high alert and stepped up security around all foreign missions, fearing an Islamist backlash, possibly after weekly prayers tomorrow.
Today, the US consulate in Berlin was partially evacuated after an employee fell ill on opening a suspicious envelope.
Bangladeshi Islamists tried to march on the US embassy in Dhaka and Iranian students protested in Tehran.
Earlier in the week, there were protests outside US missions in Tunisia, Sudan and Morocco.
The attackers were part of a mob blaming America for a film they said insulted the Prophet Muhammad. Clips of the "Innocence of Muslims," had been circulating on the internet for weeks before the protests erupted.
They show an amateurish production portraying Mohammad as a womaniser, a homosexual and a child abuser. For many Muslims, any depiction of the Prophet is blasphemous and caricatures or other characterisations have in the past provoked protests.
Mrs Clinton said Washington rejected the film's message absolutely. "It appears to have a deeply cynical purpose: to denigrate a great religion and provoke rage," she said.
An actress in the California production said the video as it appeared bore no resemblance to the original filming. She had not been aware it was about the Prophet Muhammad.
Among the assailants in Benghazi, Libyans identified units of a heavily armed local Islamist group, Ansar al-Sharia, which sympathises with al-Qaeda and derides Libya's US-backed bid for democracy.
Former Libya militant commander Noman Benotman, now president of Britain's Quilliam think-tank, said Western officials were investigating a possible link with a paramilitary training camp about 160km south of the eastern Libyan town of Derna, near the Egyptian border.
The video below shows the aftermath of the attack on the US consulate building which was set on fire
US officials said there were suggestions members of al-Qaeda's north-Africa based affiliate may have been involved.
Yemen, a key US ally, is home to al-Qaeda in the Arabian Peninsula (AQAP), viewed by Washington as the most dangerous branch of the militant network established by Osama bin Laden.
The attacks could alter US attitudes towards the revolutions that toppled secularist authoritarian leaders in Egypt, Libya and Tunisia and brought Islamists to power.
Reuters