A top Libyan commander loyal to the U.N.-brokered government says his forces have pushed into an Islamic State stronghold in central Libya and are now fighting militants on the streets of Sirte.

Col. Mohammed al-Ghasri said the advance came under the cover of heavy airstrikes. He spoke to reporters late on Thursday.

Libyan forces, mainly militias from the western city of Misrata, have been advancing on Sirte, the main IS bastion in Libya and the birthplace of former Libyan leader Mohammed Gadhafi, for the past 25 days.

Al-Ghasri says the plan is to impose a siege on the city to prevent IS militants from fleeing. He added that Sirte is a warzone and warned civilians to stay away from the troops or they would be considered a target.