CAIRO — An umbrella group for eastern Libya’s extremist militias announced Wednesday that it had overrun three more army bases in the eastern city of Benghazi and seized large amounts of heavy weapons, including armored vehicles, as it tightened its grip on the city.

The Benghazi Revolutionary Shura Council, composed of extremist militias such as Ansar al-Shariah, posted pictures of its leaders posing in front of tanks, multiple rocket launchers, and artillery they claimed to have seized from the bases they overran.

The pictures could not be independently verified, but in the past weeks the militants have seized several other bases belonging to the remnants of the Libyan army. An official in the city confirmed the recent victories by the militias and said they were now shelling the army’s remaining strongholds on the outskirts of the city.

Ansar al-Shariah is branded a terrorist organization by the U.S. and is accused of orchestrating the deadly assault on the U.S. mission in Benghazi in 2012 that left four Americans including Ambassador J. Christopher Stevens dead.

The Islamist militias began a counteroffensive after units loyal to Gen. Khalifa Hifter attempted to dislodge them from the city months earlier.

Despite support from several army units, including the elite special forces, Hifter’s troops have largely been driven out of the city and are now cornered in the Benina airport outside the city.

The security official said the militias have been pounding the airport with rockets. He spoke on condition of anonymity because he was not authorized to speak to the press.

Hifter’s campaign announced in May has upset the balance of power in Libya, where militias formed to overthrow Moammar Gadhafi in 2011 hold most of the weapons.

The militias have chosen different sides in the conflict and are now squaring off against one another across the country. Tripoli’s airport has been severely damaged after the Islamist-allied militia from the city of Misrata attacked the militia from Zintan that is holding it.