BENGHAZI, Libya – 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 that they had seized control of the sprawling convention center that was used as IS’s headquarters in the coastal city. The fighters, who are mainly from the nearby city of Misrata, launched their offensive against IS in June. They also said that they had seized the city’s main hospital of Ibn Sina from IS militants.

A statement posted on the forces’ Facebook page declared that “Sirte is returning to Libya.”

The government-supported operation, known as al-Bonyan al-Marsous, also said that it lost contact with one of its warplanes and the pilot. In an online statement, IS 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-IS fighters who have been killed in the past 24 hours.

Since Aug. 1, U.S. warplanes have launched a series of airstrikes targeting IS positions in the city. The US Africa Command (AFRICOM) said in a press release on Tuesday that the total number of strikes has reached 29 and added, “these actions, and those we have taken previously, will help deny Daesh a safe haven in Libya from which it could attack the United States and our allies.” It used an Arabic acronym for the Islamic State group.

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

IS militants seized control of the city, the hometown of Libya’s former dictator Muammar Gadhafi, in 2015.

Libya descended into chaos following Gadhafi’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 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 IS in Sirte are under the command of Serraj’s government.