|US ambassador to Libya Christopher Stevens. [Photo:BBC]|
The US state department has only confirmed the death of one official - but it has not named them.
Protesters have also attacked the US embassy in Cairo over the film.
In the attack in Benghazi, unidentified armed men stormed the grounds, shooting at buildings and throwing handmade bombs into the compound.
Security forces returned fire but Libyan officials say they were overwhelmed.
A Libyan official has said Ambassador Stevens died from suffocation as a result of the attack.
In a statement earlier, Secretary of State Hillary Clinton confirmed the death of a US official, saying: "We are heartbroken by this terrible loss".
"Some have sought to justify this vicious behaviour as a response to inflammatory material posted on the Internet," she said in a statement.
"The United States deplores any intentional effort to denigrate the religious beliefs of others. But let me be clear: There is never any justification for violent acts of this kind."
Reports say a militia known as the Ansar al-Sharia brigade was involved in the attack, but the group has denied the claim, the BBC's Rana Jawad in Tripoli says.