CAIRO — Progovernment Libyan militias backed by the United States said Wednesday they had seized the Islamic State’s last stronghold in the country, in the seaside city of Sirte. If confirmed, the capture would be a severe blow to the militant organization’s expansion into North Africa.

Militia announcements quoted by Libyan news agencies and television outlets said the militia fighters were still hunting remnants of the Islamic State forces hiding in residential neighborhoods in Sirte.

But the militias claimed to have taken the heavily fortified Ouagadougou Center, which the Islamic State had used as its headquarters.

In a statement broadcast on Misrata TV, a station based in the nearby city of Misrata, Mohamed al-Ghassri, a spokesman for the attacking militia force, said the Ouagadougou Center and a nearby hospital had been captured.

The center had underground bunkers and fortifications dating from the era of Moammar Gadhafi, the longtime leader of Libya overthrown nearly five years ago.

The Islamic State’s loss of Sirte would signify the culmination of a summer-long offensive by militias from Misrata, under the auspices of the Government of National Accord.

Over the last 10 days, the militias have been supported by heavy US airstrikes, using drones based in Jordan. The US Africa Command has reported 28 airstrikes from the beginning of that campaign, Aug. 1, to Aug. 8.

The Islamic State had held Sirte for the past year. Its occupation of the city represented the organization’s most brazen expansion from its power bases in Iraq and Syria.

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

Promilitia factions also reported that a Libyan air force warplane had been shot down by Islamic State fighters in Sirte on Wednesday.

The territory seized by the Islamic State in Libya had been considered the most important of the group’s overseas wilayats, or provinces.

As early as October 2014, extremists in the Libyan city of Darnah pledged allegiance to the Islamic State, and a month later, the Islamic State leader, Abu Bakr al-Baghdadi, named Libya as one of the group’s official provinces.

That province was eventually centered in Sirte, which became the axis of the Islamic State’s power in Libya.

The organization sought to give its Libya province the trappings of a state, modeled after the one it was trying to run in Iraq and Syria. Early on, senior Islamic State members arrived by boat to help administer the territory, creating a degree of connective tissue that has mostly been lacking in other areas the group has seized.