Demonstrators attacked the U.S. embassies in Yemen and Egypt on Thursday in protest at a film they consider blasphemous to Is lam, and th e United States sent war ships towards Libya, w h e re the U.S. ambassador was killed in related violence this week.

In Libya, authorities said they had made four arrests in the investigation into the attack that killed ambassador Christopher Stevens and three other Americas in Benghazi on Tuesday.

U.S. President Barack Obama, facing a new foreign policy crisis less than two months before seeking re- e lection, has vowed to bring to justice those responsible for the Benghazi attack, which U.S. officials said may have been planned in advance - possibly by an al Qaeda-linked group.

U.S. Secretary of State Hillary Clinton said Washington had nothing to do with the crudely made film posted on the Internet, which she called "disgusting and reprehensible."

The amateurish production, entitled the "Innocence of Muslims," and originating in the United States, portrays 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 violent protests across the Muslim world.

Demonstrations spread further on Thursday, with U.S. embassies again the targets of popular anger among Muslims questioning why the United States has failed to take action against the makers of the film.

Hundreds of Yemenis broke through the main gate of the heavily fortified U.S. embassy compound in Sanaa, shouting "We sacrifice ourselves for you, Messenger of God." They smashed windows of security offices outside the embassy and burned cars.

A security source said at least 15 people were wounded, some by gunfire, before the government ringed the area with troops.

In Egypt, protesters hurled stones at a police cordon around the U.S. embassy in Cairo after climbing into the compound and tearing down the American flag. The state news agency said 13 people had been hurt in violence since late on Wednesday.

Around 200 demonstrators gathered outside the U.S. embassy in Kuwait and hoisted banners, one of which bread in English: "USA stop the bullshit. Respect us."

In Bangladesh, Islamists tried to march on the U.S. embassy in Dhaka and Iranian students protested in Tehran. Earlier in the week, there were protests outside U.S. missions in Tunisia, Morocco and Sudan an d stat e-backed Islamic scholars in Sudan have called a mass protest after Friday prayers.

The U.S. ambassador to Libya was killed during a protest against the film when Islamists armed with guns, mortars and grenades staged military-style assaults on the Benghazi mission.

A Libyan doctor said Stevens died of smoke inhalation. U.S. information technology specialist Sean Smith also died at the consulate, while two other Americans were killed when a squad of security personnel sent by helicopter from Tripoli to rescue diplomats from a safe house came under mortar attack.

Of the latter, one was identified by his family as Glen Doherty, 42, a f ormer Navy SEAL w ho worked as a private security contractor. U.S. media identified the other man as another former navy SEAL - Tyrone Woods, aged 41.

In an interview with the U.S. network ABC News last month Doherty said he was w orking with the State Department on an intelligence mission to round up a nd destroy s houlder-fired surface-to-air missiles.

Thousand of these missiles went missing in Libya after Muammar Gaddafi's overthrow by in a U.S.-backed uprising last year, prompting concerns that they could end up in the hands of al Qaeda militants.

FIRST U.S. AMBASSADOR KILLED SINCE 1979

Stevens, 52, had spent a career operating in perilous places, mostly in the Arab world, and became the first American ambassador killed in an attack since Adolph Dubs, the U.S. envoy to Afghanistan, died in a 1979 kidnapping attempt.

Tuesday's incident, on what was the 11th anniversary of al Qaeda's attacks on the United States on Sept. 11, 2001, has created a w orrying f oreign policy crisis for Obama ahead of his re-election bid.

Speaking at a campaign rally in Colorado on Thursday, Obama said he had ordered his administration to do whatever was necessary to protect Americans abroad and that aides had been in contact with other governments "to let them know they've got a responsibility to protect our citizens."

The U.S. military has dispatched two destroyers toward the Libyan coast, in what an official said was a move to give the administration flexibility for any future action. The USS Laboon, was already in position and the other destroyer, the USS McFaul, was at least a day away, a U.S. official said.

The U.S. military also sent a Marine Corps anti-terrorist team to boost security in Libya.

Republican Mitt Romney, Obama's challenger, criticised the president's response to the crisis earlier this week, saying it was disgraceful to be seen to be apologising for American values of free speech.

Obama's campaign accused Romney of trying to score political points at a time of national tragedy and the challenger also provoked some criticism from within his own party.

Campaigning on Thursday, Romney stopped short of repeating the charge, but said that under Obama the United States seemed to be at the mercy of world events rather than shaping them, and argued for a stronger military, at a time when U.S. armed forces are facing an unprecedented budget squeeze.

The administration again sought to calm Muslim tensions on Thursday.

Clinton said Washington rejected the film's message absolutely, and added: "It appears to have a deeply cynical purpose: to denigrate a great religion and provoke rage."

At the same time, Clinton noted U.S. religious tolerance and commitment to freedom of speech and said there was no justification for violence.

"I know it is hard for some people to understand why the United States cannot or does not just prevent these kinds of reprehensible videos from ever seeing the light of day," she said. "I would note that in today's world, with today's technologies, that is virtually impossible.

"But even if it were possible our country does have a long tradition of free expression which is enshrined in our constitution and our law. And we do not stop individual citizens from expressing their views no matter how distasteful they may be."

AL QAEDA LINK SEEN

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 U.S.-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 100 miles (160 km) south of the eastern Libyan town of Derna, near the Egyptian border.

U.S. officials said there were suggestions members of al Qaeda's North Africa-based affiliate may have been involved.