Ambassador Christopher Stevens is said to be among four US officials killed in a protest over a US-produced film that is said to insult the Prophet Muhammad.
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.
Our correspondent says many people are still armed following the conflict that overthrew Col Muammar Gaddafi last year.
The film that sparked the demonstration is said to have been produced by a 52-year-old US citizen from California named Sam Bacile, and promoted by an expatriate Egyptian Copt.
The two men are described as having anti-Islamic views.
A trailer of the low-budget movie has appeared on YouTube translated into Arabic.
There were calls on social media networks for protests against US interests in the capital, Tripoli, but no disturbances have been confirmed, our correspondent says.
The rally followed a demonstration in Cairo, in which protesters breached the US embassy and tore down the US flag, which was flying at half mast to mark the 9/11 attacks, and replaced it with an Islamist banner.
Thousands of protesters had gathered outside the US embassy in the Egyptian capital.
Egyptian protesters condemned what they said was the humiliation of the Prophet of Islam under the pretext of freedom of speech.
“Both Muslims and Christians are participating in this protest against this offence to Islam,” said one protester, according to Associated Press news agency.
– BBC News