Protesters broke in to the US embassy in Yemen's capital overnight as anger over a low-budget US film which mocks Islam and the Prophet Mohammed swept the Muslim world.

Reports say as many as four protesters died when Yemeni police used live ammunition against demonstrators who had breached the wall of the embassy in Sana'a.

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The US said all its embassy staff were safe.

Protests against the film, titled Innocence of Muslims, are growing throughout the Middle East and North Africa, two days after four Americans, including ambassador Chris Stevens, were killed in an attack on the US consulate in Benghazi.

In Egypt, president Mohamed Morsi condemned the film but called for calm as angry protesters throwing stones and bottles clashed with security outside the US embassy in Cairo.

In Kuwait, about 500 demonstrators gathered near the US embassy waving a black Al Qaeda flag.

Demonstrations were also reported in countries including Tunisia, Yemen, Lebanon, Gaza, Morocco, Sudan, Iran, Iraq, Bangladesh and Mauritania.

In Pakistan, authorities blocked access to the Innocence of Muslims trailer on YouTube while tightening security around US diplomatic missions.

In Libya, officials said they had made four arrests in relation to the attack on the Benghazi consulate, in what the country's new prime minister said was a "big advance" in the case.

The shooting in Sana'a came as protesters, chanting "O, messenger of Allah... O, Mohammed" launched a second charge on the complex which they had stormed earlier, but had been ejected from by the security forces.

Yemen president Abdrabuh Mansur Hadi apologised to his US counterpart Barack Obama and the American people for the acts of a "mob", and ordered an investigation.

"Those who are behind [the attack] are a mob that are not aware of the far-reaching plots of Zionist forces, especially those who made a film insulting the prophet," he said.

Protests spread

Sorry, this video has expired Middle East Institute professor Abdallah Schleifer discusses the continuing clashes

In other Muslim countries, security measures have tightened around US missions and several governments tried to block internet access to the controversial film.

Egyptian police used tear gas as they clashed with crowds protesting outside the US embassy in Cairo.

Egypt's Islamist president Mr Morsi slammed the "attacks" on the Prophet Mohammed in the film, while also stressing that he condemned the violence.

"We Egyptians reject any kind of assault or insult against our prophet. I condemn and oppose all who... insult our prophet," Mr Morsi said in remarks broadcast by state television in reference to the controversial film.

"(But) it is our duty to protect our guests and visitors from abroad."

In Iran, up to 500 people protested in Tehran, chanting "Death to America!" and death to the movie's director.

The rally took place near the Swiss embassy, which handles US interests in the absence of US-Iran diplomatic ties.

Fearing a violent backlash, Afghan president Hamid Karzai postponed a planned visit to Norway.

Meanwhile, authorities in Pakistan have stepped up security measures around US diplomatic missions.

Khurram Rasheed, a senior police official responsible for diplomats' security in Islamabad, said the US embassy was in the city's heavily-fortified diplomatic enclave, and the entrance would be closed and extra security forces deployed.

In the eastern city of Lahore, police said they had issued an alert to protect the US consulate.

In Iraq, thousands of supporters of powerful Shiite cleric Moqtada al-Sadr protested in several cities.

Thousands of people, including some MPs from Sadr's parliamentary bloc, marched for about an hour in the Sadr City area of north Baghdad.

Protesters burned an American flag and carried banners including one that read, "Death to America, enemy of nations."

In the Tunisian capital on Wednesday, police fired tear gas to disperse a crowd of several hundred.

Protests against the film were also held on Wednesday outside US missions in Morocco and Sudan.

Innocence of Muslims, in which actors have strong American accents, portrays Muslims as immoral and gratuitously violent.

It pokes fun at the Prophet Mohammed and touches on themes of paedophilia and homosexuality, while showing him sleeping with women, talking about killing children and referring to a donkey as "the first Muslim animal".

Mystery has deepened over the film, with conflicting accounts from backers and promoters but no one owning up to having actually directed it.

US media initially cited someone claiming to be an American-Israeli and calling himself Sam Bacile as saying he made the film on a $5 million budget with the help of 100 Jews, but no record of such a person has been found.

A car burns during protests at the US embassy in Sana'a, Yemen. ( Reuters: Mohamed al-Sayaghi )

Consulate killing probe

Libyan interior ministry spokesman Abdelmonem al-Horr said an independent judicial committee has also been set up to carry out an inquiry into the killings in Benghazi.

Mr Horr said the inquiry would be "very complicated" because the crowd outside the consulate during the attack had been very mixed.

"There were extremists, ordinary citizens, women, children and criminals," he said.

"There were also shots fired from a nearby farm. We need time to determine who was responsible."

Libya has apologised to Washington over the attack, which was discussed in a telephone conversation between US president Obama and Mohamed al-Megaryef, president of Libya's highest political authority, the General National Congress.

US officials now believe the attack was not the chaotic anti-American demonstration as they had initially thought, but rather an organised ambush carried out by at least 20 men, lasting five hours.

Secretary of state Hillary Clinton said: "This was an attack by a small and savage group, not the people or government of Libya."

Mr Obama assured Americans justice would be done but said the relationship between Libya and the US would not be damaged by the incident.

"This attack will not break the bonds between the United States and Libya," he said.

"Libyan security personnel fought back against the attackers alongside Americans.

"Libyans helped some of our diplomats find safety and they carried Ambassador Stevens' body to the hospital, where we tragically learned that he had died."

The US has sent a detachment of 50 US Marines to secure the American embassy in the capital Tripoli, where staff numbers were being cut to emergency levels.

Washington also began evacuating all its staff from its mission in Benghazi while at the same time sending two destroyers to "the vicinity of Libya" as a precautionary measure, a senior US official said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 4 minutes 7 seconds 4 m 7 s US officials now believe their ambassador to Libya was killed by an organised ambush

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