Libya said Thursday it has made arrests and opened a probe into an attack on the US consulate in Benghazi that killed the American ambassador, amid speculation that Al-Qaeda rather than a frenzied mob was to blame.

"The interior and justice ministries have begun their investigations and evidence gathering and some people have been arrested," the deputy interior minister, Wanis al-Sharif, told AFP.

He declined to give any details of the number of people in custody or their backgrounds "so as not to hamper the smooth running of the investigation."

The announcement of the inquiry comes as protests against a low-budget, privately produced film denigrating the Prophet Mohammed -- the spark of Tuesday's attack on the Benghazi mission -- spread across the Arab world.

"All measures are being taken. An independent judicial committee has been set up to carry out an inquiry," Abdelmonem al-Horr, spokesman for the interior ministry's security commission, told AFP.

Tuesday's assault on the consulate was initially believed to have been motivated by outrage over an amateur Internet film made in America that insulted Islam, but US officials later said it might have been a planned, pre-meditated assault by Al-Qaeda affiliates or sympathisers.

It began when a mob stormed the consulate but later, according to witnesses, armed hardliners aimed rocket-propelled grenades at the complex before setting it alight, killing Ambassador Chris Stevens and three other Americans.

Horr said the inquiry will be "very complicated" because the crowd outside the consulate 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 Barack Obama and Mohamed al-Megaryef, president of Libya's highest political authority, the General National Congress.

US officials said Obama thanked Megaryef for expressing condolences while also urging Libya to work with US authorities to bring those behind the attack to justice.

"He also expressed appreciation for the cooperation we have received from the Libyan government and people in responding to this outrageous attack, and said that the Libyan government must continue to work with us to assure the security of our personnel," a White House statement said.

US officials said a detachment of 50 Marines had been dispatched 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.

"Make no mistake, we will work with the Libyan government to bring to justice the killers who attacked our people," Obama said in a Washington address, while ordering increased security at US diplomatic missions around the world.

-- Smoke inhalation --

The assault on the US consulate in Benghazi followed a violent protest at the American embassy in Cairo over an amateurish anti-Islamic film made in the United States and reportedly promoted by a group of US-based Egyptian Copts.

Initial reports said Stevens and the three other Americans were killed by an angry mob as they tried to flee in a car.

But it is now believed Stevens died from smoke inhalation after becoming trapped in the compound when suspected Islamic militants fired on the building with rocket-propelled grenades and set it ablaze.

The finger of blame initially fell on hardline Sunni Islamists of the Salafist group Katibat Ansar al-Sharia (Brigade of the Supporters of Sharia).

But in a statement on Thursday, the group condemned "the accusations without any verification or investigation" which had emerged against it in the Libyan media.

US officials are investigating the possibility that the assault was a plot by Al-Qaeda affiliates or sympathisers, using the protests as a diversion to carry out a coordinated revenge attack on the 11th anniversary of 9/11.

Fresh protests against the film erupted outside the US embassy in Cairo on Thursday, where stone- and bottle-throwing demonstrators were met by riot police firing tear gas.

In Yemen, demonstrators briefly broke into the US embassy compound before bring expelled by police, an AFP correspondent reported.

Police opened fire when protesters made a second charge on the Sanaa complex, killing one demonstrator and wounding five, a security official said.