By Aidan Lewis

TRIPOLI (Reuters) - Libyan forces fighting Islamic State in its stronghold of Sirte said on Wednesday that 36 of their men had been killed and nearly 150 wounded in the previous day's clashes, one of the heaviest tolls in their month-long campaign.

Islamic State militants had been fighting hard to defend the shrinking territory they still control in the residential center of Sirte, said Abdalla Binrasali, a spokesman at the forces' media center in Misrata.

"The resistance was fierce and they were firing with everything they've got, mortars, rockets and rifles," he said. "They fear that if they lose more ground they will be defeated."

Brigades largely composed of fighters from Misrata launched a campaign to retake Sirte from Islamic State last month. They rapidly recaptured ground west of Sirte at the end of May, but their advance slowed as they closed in on the center of the coastal city.

On Tuesday fighting escalated and the brigades said they had taken control of parts of the "700" neighborhood, the broadcasting and electricity company headquarters and a mosque.

The "700" neighborhood is strategically important because Islamic State snipers have been positioning themselves on the district's taller buildings.

The brigades based in Misrata are aligned with a U.N.-backed unity government that arrived in Tripoli in March. It is seeking to replace two other rival governments that were set up in Tripoli and the east in 2014, and to unite Libya's many political and armed factions.

Islamic State took advantage of Libya's political turmoil to establish a presence in several of the country's towns and cities from 2014.

It took full control of Sirte, the hometown of former Libyan leader Muammar Gaddafi, last year, creating its most significant base outside Syria and Iraq. However, the jihadist group has struggled to retain territory elsewhere in Libya.

(Editing by Robin Pomeroy)