At least 16 Libyan militia fighters have been killed in two attacks by the Islamic State group as pro-government forces try to dislodge the extremists from Sirte, their last stronghold in the country, officials said on Thursday.

The bombings were part of a wave of suicide attacks launched by IS to stall the initially fast progress of the anti-IS forces, who are led by militias from the western city of Misrata.

IS counter-attacks have killed nearly 180 militia fighters since the offensive began in early May.

In recent weeks, the militias have managed to take over large areas of Sirte, including the air base, the port, and a number of barracks. Videos and pictures from the city showed militiamen knocking down a central stage used by IS militants to behead and shoot opponents. Officials said that they have cornered the militants inside a small area of town around the sprawling convention center that the group turned into its headquarters.

However, some of the militants appear capable of striking from behind front lines, possibly by disguising themselves among the families escaping the fighting, said Ahmed Hadia, the head of the media center of the anti-IS operation.

On Thursday, the spokesman of Misrata hospital, Abdel-Aziz Essa, told The Associated Press that 10 fighters were killed and seven injured when a suicide bomber struck a police station in the early hours in Abu Grain, a village located on a crossroad that connects the coastal cities with the south.

A day earlier, Hadia said that the operation room has lost contact with a six-member scouting unit inside Sirte. Hours later, IS posted pictures online of the bodies of young men in uniform in the back of a pick-up truck, and described them as members of the Misrata forces. Hadia said that two of the men were identified as belonging to their forces.

He said that the IS fighters who had managed to escape Sirte "could be a more serious threat than the fighters we are currently surrounding."

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Associated Press writer Maggie Michael contributed to this report from Cairo.