Libyan authorities have made four arrests in the investigation into the attack on the US consulate in Benghazi in which the US ambassador and three embassy staff were killed, the deputy interior minister said.
"Four men are in custody and we are interrogating them because they are suspected of helping instigate the events at the US consulate," Wanis Sharif told the Reuters news agency on Thursday.
He gave no further details.
US ambassador to Libya Christopher Stevens and three other Americans died after the gunmen attacked the US consulate and a safe house refuge in the eastern city of Benghazi on Tuesday night.
The bodies of the four men are being flown to the US and are due to arrive at the Andrews Air Force base, where President Barack Obama and Secretary of State Hillary Clinton are expected to speak at a ceremony in honour of the diplomats.
The attackers were part of a mob blaming America for a film they said insulted the Prophet Mohammad.
Demonstrators attacked the US embassies in Yemen and Egypt on Thursday in protests against the film, and American warships were moved closer to Libya.
Barack Obama, the US president, has pledged to bring to justice those responsible for the Benghazi attack, which US officials said may have been planned in advance. Secretary of State Hillary Clinton said Washington had nothing to do with the video, which she called "disgusting and reprehensible".
A "big advance" has been made in the investigation into the deadly attack, Mustafa Abu Shagur, the Libyan prime minister, told the AFP news agency in an exclusive interview.
"We have made a big advance," Abu Shagur said in his first interview since his election as premier on Wednesday night.
"We have some names and some photographs. Arrests have been made and more are under way as we speak."
The new prime minister did not elaborate on how many suspects were in custody or what groups, if any, they were connected to.
Death of US ambassador to Libya
Roving correspondent, reporting from Benghazi
“We still don’t have an official version of who is behind the attacks, but I did speak to several eye-witnesses and all of them told me that four vehicles entered the grounds of the US consulate, and there were heavily armed men. One witness said they had black flags, meaning the flags of al-Qaeda, and that the protesters were really outside of the grounds of the consulate.
The consulate consists of four one-storey buildings inside a vast compound, and all four of them are completely torched.
There are holes from rockets on the walls of the buildings, especially the one where the ambassador was holed up in. We understand the attackers concentrated their attacks mainly on that building.
Eye-witnesses also told us that it was only afterwards that the protesters came in, and smashed the windows and grabbed the ambassador from inside, and got him out from the room [he was holed up in]. We were told he was alive then, and that he died either on his way to the hospital or at the hospital.
From the amount of destruction around, it seems to be a well co-ordinated attack.”
"We don't want to categorise these people until we know all the facts," he said.
Shaqur was similarly reserved about going into details when he spoke to AFP earlier on Thursday.
"The interior and justice ministries have begun their investigations and evidence gathering and some people have been arrested," he said.
Initial reports said Stevens and the three other Americans were killed by a mob outside the consulate in the eastern city of 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 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 co-ordinated revenge attack on Tuesday's anniversary of the September 11, 2001 attacks in the US.
Libyan authorities initially pointed a finger of blame at supporters of Muammar Gaddafi, who was toppled and slain in an uprising last year that was backed by NATO air power, and at al-Qaeda.
But Abu Shagur played down the al-Qaeda line.
"We don't have any proof as yet of an al-Qaeda presence as an organisation in Libya," although "some youths have been influenced by the extremist ideology of al-Qaeda," he said.
Abu Shagur said "extremists" were a tiny minority in Libya who "do not number more than 100 or 150," whereas most of the youth in the country were moderates.
The attack on the US consulate was "a cowardly, criminal and terrorist act," he said, adding it was "isolated, not representing a phenomenon in Libyan society and it will not have negative consequences with our allies" who backed the revolution.
He did not have confirmation that the US was sending two warships off the Libyan coast. "But we will not accept anyone entering inside Libya. That would infringe on sovereignty and we will refuse," the prime minister said.
A decision to deploy a team of 50 US Marines was taken "in co-ordination with Libyan authorities," he added. They would guard the US embassy in Tripoli and two diplomatic residences.