Libya's pro-government forces have seized control of the key port of Sirte from Islamic State, helped by British and American support.

Many IS leaders have fled to the open desert south of Sirte, although some are still holed up in the city centre.

Brigadier General Mohammed al-Ghasri confirmed that foreign army experts including Americans and British are providing support to the forces battling the IS militants, also known as Daesh.

Fighters from forces aligned with Libya's new unity government are seen stationed behind the barricade as they advance on Sirte. Photo: Reuters

"The British and US experts are helping us with logistical and intelligence to deal with Daesh suicide bombers and with tactical and strategic planning," he told The Associated Press.

Earlier, a media official said that the forces have found bodies of militants, handcuffed and shot in the head, inside army barracks in the city.

Ahmed Hadiya told the AP that at least six bodies were found inside the al-Saadi barracks after the facility was overrun by the forces allied to the UN-brokered government.

He said he believes the bodies are of IS "militants who wanted to flee".

He added that the offensive to capture the Mediterranean city is nearing its "final stage" and that some IS fighters are cornered inside a sprawling convention centre and preparing for the final showdown.

The fighting, which started a month ago, is led by militias from the western city of Misrata, west of Sirte, who advanced from the western and southern fronts.

IS took over Sirte last year exploiting Libya's turmoil to gain a foothold in the North African country.