SIRTE, Libya (Reuters) - Libyan forces said on Friday they had edged further into the center of Sirte as they seek to recapture the city from Islamic State, following heavy fighting until late the previous evening that left dozens dead.

Forces aligned with Libya's United Nations-backed government in Tripoli advanced rapidly on the militant group's Libyan stronghold in May, but they have faced resistance from snipers, suicide bombers and mines as they have closed in on the city center.

Sirte had been controlled by Islamic State since last year, becoming its most important base outside Syria and Iraq, and its loss would be a major setback for the group.

After a lull in fighting earlier this week, the government-backed forces launched a fresh assault on several fronts after first pounding IS positions with artillery and air strikes.

The brigades, made up mainly of fighters from the western city of Misrata, said in a statement that they had captured a hotel on the eastern front line used by Islamic State snipers, and also taken control of part of the "Dollar" neighborhood.

They said they had foiled three attempted car bomb attacks and destroyed an armored vehicle. A Reuters witness saw a tank belonging to the brigades being blown up, though it was not clear what caused the explosion.

The witness said shelling continued late into Thursday night, but it was quiet on Friday morning.

Nearly 50 bodies of Islamic State fighters killed during Thursday's clashes were counted, the statement from the government-backed forces said. At least 25 brigade members were killed and 200 wounded, according to Misrata's central hospital.

Since the campaign for Sirte began in May, more than 300 fighters from the brigades have been killed and more than 1,300 wounded, a spokesman for the forces said.

Islamic State expanded into Libya amid the political chaos and security vacuum that developed after long-time ruler Muammar Gaddafi was toppled in an uprising in 2011.

The group extended its presence along about 250 km (155 miles) of Libya's coastline, but failed to win and retain territory elsewhere in the country.

(Reporting by Goran Tomasevic and Ahmed Elumami; Writing by Aidan Lewis; Editing by John Stonestreet)