TRIPOLI (Reuters) - Libyan forces resumed their advance on Thursday against Islamic State militants holding out in a few streets in their former stronghold of Sirte, saying they had captured 25 houses and a stash of arms.

Backed by U.S. air strikes, the Libyan brigades have reduced the area held by the jihadists to a small patch of land near Sirte's Mediterranean seafront after a campaign of more than six months.

Lately, they say, they have been advancing with more caution in the Ghiza Bahriya district to limit casualties among their own fighters and among any hostages and families still held there.

"According to our information there are still civilians including women and children inside," Mohamed al-Gasri, a spokesman for the brigades, told private TV station Libya's Channel. "The instructions are not to rush in, in order to limit the damage."

Several groups of civilians have either escaped or been released in Sirte in recent weeks, including migrants from sub-Saharan Africa who were captured by Islamic State as they tried to make their way toward Europe.

It is not clear how many militants or civilians remain. Earlier this week Libyan forces said they had counted the bodies of several dozen slain Islamic State fighters.

The Libyan forces are led by brigades from the western city of Misrata and loosely aligned with a U.N.-backed government in Tripoli. They have faced fierce resistance from Islamic State snipers as well as suicide bombs and hidden explosives, losing more than 670 of their men since their campaign began in May.

Rida Issa, another spokesman for the brigades, said they had recovered a 106 mm anti-tank gun and a store of ammunition during Thursday's advance. One of the Misrata-led forces had been killed and three wounded, he said. He said perhaps 70 houses remained under Islamic State control.

As of Tuesday, the United States had carried out 420 air strikes in Sirte since Aug. 1, according to U.S. Africa Command.

(Reporting by Aidan Lewis and Ahmed Elumami; editing by Mark Heinrich)