Over the last 10 days, the militias fighting the Islamic State in Libya have been supported by heavy American airstrikes, using drones based in Jordan. The United States Africa Command has reported 28 airstrikes from the beginning of that campaign, Aug. 1, to Monday.

The Islamic State, also known as ISIS or ISIL, had held Surt for the past year. Its occupation of the city represented the organization’s most brazen expansion from its power bases in Iraq and Syria.

While the American military did not specify exactly where its airstrikes had been aimed, it is believed that they were concentrated in and around Surt. The militias’ offensive against the Islamic State had reduced the area they controlled from 150 miles of coastline to the area immediately around the city.

The birthplace of Colonel Qaddafi, Surt is also where the Libyan dictator was killed by antigovernment militia fighters in 2011.

Officials at the Pentagon said they could not confirm that the Islamic State’s headquarters in Surt had fallen, but one senior official, speaking on condition of anonymity under military rules, said he had no reports suggesting the militia claims were untrue.

Libya’s hodgepodge of militias, answering to three different factions claiming to control the country, have often been prone to exaggerated claims.

Pro-militia factions also reported that a Libyan Air Force warplane had been shot down by Islamic State fighters in Surt on Wednesday.