U.S.-backed Libyan forces said on Wednesday they have taken over the Islamic State group's headquarters in Sirte, the militants' final bastion in Libya, breaking a weeks-long stalemate with the help of U.S. airstrikes.

The fighters said they seized control of the sprawling convention center that was used as the terror group's headquarters in the coastal city. They also said that they had seized the city's main hospital of Ibn Sina from ISIS.

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

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

U.S. warplanes have launched a series of airstrikes targeting ISIS positions in the city. The air support came in response to a request for assistance from Libya's U.N.-brokered government after battles in Sirte stalled.

The government-supported operation, known as al-Bonyan al-Marsous, also announced that it lost contact with one of its warplanes and the pilot. In an online statement, ISIS claimed responsibility for shooting down the plane and killing the pilot.

Mohammed Shamia, a spokesman for the operation, posted on his Facebook page a list of 14 anti-ISIS fighters who have been killed in the past 24 hours.

The militants seized control of the city, the hometown of Libya's former dictator Muammar Qaddafi, in 2015.

Since Aug. 1, U.S. warplanes have launched a series of airstrikes targeting IS positions in the city.  The U.S. Africa Command (AFRICOM) said in a press release on Tuesday that the total number of strikes has reached 29.

The air support came in response to a request for assistance from Libya's U.N.-brokered government after battles in Sirte stalled.

Libya descended into chaos following Qaddafi's ouster in 2011. The country has been divided between warring parliaments and governments, with each backed by a loose array of militias and tribes.

In December of last year, the United Nations struck a deal to unify the country's rival governments and created a third unity government led by Fayez Serraj, who still needs a crucial vote of confidence from the eastern-based parliament.

The forces fighting ISIS in Sirte are under the command of Serraj's government.

The Associated Press contributed to this report.