"We received orders on Thursday evening to immediately suspend all flights for security reasons," the source told AFP.

Initial reports said that Ambassador Chris Stevens and three other Americans were killed by a mob outside the consulate in Benghazi on Tuesday as they tried to flee an angry protest against a US-produced movie deemed offensive to Islam.

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.

US officials identified two of the four Americans killed in Tuesday's attack as former members of the elite Navy SEALs, Tyrone Woods and Glen Doherty, while the other victim was named as Sean Smith, an information management officer.

US officials are investigating the possibility that the assault was a plot by Al-Qaeda affiliates or sympathisers, using the protest against the film as a cover to carry out a coordinated revenge attack on Tuesday's anniversary of the September 11, 2001 attacks in the United States.

Prime Minister Mustafa Abu Shagur told AFP on Thursday that a"big advance" had been made in the probe into the deadly attack.

In his first interview since his election as premier on Wednesday night, Abu Shagur said that arrests had already been made and that more were under way.

"We have made a big advance. We have some names and some photographs," he said.