The media center for the Libyan militias battling Islamic State militants in the group's last remaining bastion in the North African country have seized a radio station used by the extremists and a mosque near their headquarters.

It says in a statement that 18 militiamen and dozens of IS militants were killed in "fierce" fighting on Tuesday. It says the radio station used to broadcast speeches by IS leader Abu Bakr al-Baghdadi.

Militias from the nearby city of Misrata launched an assault last month on Sirte, the only city outside of Iraq and Syria to have been taken over by IS. The militiamen rapidly advanced into the city but the offensive has bogged down in recent days as IS has carried out a series of suicide bombings.