BENGHAZI, Libya -- U.S.-backed Libyan forces said Wednesday that they had taken over the Islamic State militant group's headquarters in Sirte, breaking a weekslong deadlock with the help of U.S. airstrikes.
The Libyan fighters said they had seized control of the convention center that was used as the Islamic State's headquarters in the coastal city. The fighters, who are mainly from the nearby city of Misrata, began their offensive against the militant group in June. They also said they had seized the city's main hospital.
A statement posted on the forces' Facebook page declared that "Sirte is returning to Libya."
The government-supported operation also said Wednesday that it had lost contact with one of its warplanes and the pilot. In an online statement, the Islamic State 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-Islamic State fighters who had been killed in the previous 24 hours.
Since Aug. 1, U.S. warplanes have launched airstrikes targeting Islamic State positions in the city. The U.S. Africa Command said in a news release Tuesday that the total number of strikes had reached 29.
"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," the statement said, using an Arabic acronym for the Islamic State group. The air support came in response to a request for assistance from Libya's United Nations-brokered government after battles in Sirte stalled.
U.S. special operations forces are providing direct, on-the-ground support to the Libyan fighters, U.S. and Libyan officials have said, coordinating American airstrikes and providing intelligence information.
U.S. officials, who spoke on the condition of anonymity to discuss the mission, said U.S. troops are working out of a joint operations center on the city's outskirts and that their role is limited to supporting forces loyal to the country's fragile unity government.
Robyn Mack, a spokesman for the U.S. Africa Command, said small numbers of U.S. military personnel will continue to exchange information with local forces, but declined to provide details.
In recent days, Libyan militia commanders had declared that there were no Western boots on the ground and the fight was theirs alone. But U.S. and British personnel, carrying radios and wearing black body armor and tan fatigues, were seen in Sirte this week, according to officers allied with the Libyan government.
Militants seized control of the city last year.
Libya descended into chaos after the ouster of leader Moammar Gadhafi in 2011. The country has been divided between warring parliaments and governments, with each backed by an array of militias and tribes.
In December, the U.N. struck a deal to unify the country's rival governments and create a government led by Fayez Serraj, who still needs a vote of confidence from the eastern-based parliament
The forces fighting the Islamic State in Sirte are under the command of Serraj's government.
Information for this article was contributed by Rami Musa of The Associated Press and by Missy Ryan and Sudarsan Raghavan of The Washington Post.