US-backed Libyan fighters say they have taken over the Islamic State group's headquarters in Sirte, the militants' final bastion in Libya.

The fighters said they have seized control of the sprawling convention centre that was used as IS headquarters in the coastal city.

The militia fighters, who are mainly from the nearby city of Misrata, launched their offensive against IS in June.

A statement on the forces' Facebook page declares that "Sirte is returning to Libya".

US warplanes have launched a series of air strikes targeting IS positions in the city. The air support came in response to a request for assistance from Libya's UN-brokered government after battles in Sirte stalled.

The militants seized control of the city, the home town of Libya's long-standing dictator Muammar Gaddafi, in 2015.

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