"The ambassador was killed along with three other officials," said Wanis al-Sharif, the deputy minister of the interior. Stevens' death in Tuesday's attack was confirmed in a tweet by Mustafa Abu Shagur, the deputy prime minister.
Stevens, a career officer with the US foreign service, had been in the country for less than four months after taking up his post in the capital Tripoli in May.
Fawzi Wanis, who heads the High Security Commission in Benghazi, confirmed that Stevens was at the consulate when it was attacked.
The ambassador was killed when an armed mob protesting against a film deemed offensive to Islam attacked the US mission, just hours after Islamists also stormed Washington's embassy in the Egyptian capital Cairo.
The film at the centre of the anti-US protests was made by an Israeli-American who describes Islam as a "cancer" and depicts the Prophet Mohammed sleeping with women, the Wall Street Journal reported.
Before confirmation of Stevens' death, US Secretary of State Hillary Clinton announced that a State Department official had been killed in the attack on the consulate, saying: "We are heartbroken by this terrible loss."