US ambassador to Libya Christopher Stevens and three others died when gunmen attacked the consulate and a safe house refuge in the eastern city on Tuesday night. The attackers were part of a crowd blaming America for a film they said insulted the Prophet Mohammad.

So far four people have been arrested and are being questioned, Libyan officials have said.

"We know of 50 people who were involved in the attack, we have names and we know who they are, but there could be more," Abdel-Monem Al-Hurr, spokesman for Libya's Supreme Security Committee, said.

"Four have been arrested. Some of the others may have escaped via Benghazi airport, maybe to Egypt, but this not confirmed. We have given their names to all of the Libyan border entry points."

A spokesman for President Barack Obama on Friday said officials had no evidence the attack was pre-planned, an assertion which has added to confusion over the incident.

Immediately after the attack, US officials, speaking on condition of anonymity, were quoted widely in the media saying they believed the attack was well-planned and organised.