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 centre.

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 last week, the government-backed forces launched a fresh assault on several fronts after first pounding ISIS positions with artillery and air strikes.

The brigades, made up mainly of fighters from the western city of Misrata say they have captured a hotel on the eastern front line used by ISIS snipers, and also taken control of part of the "Dollar neighbourhood."

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.

ISIS 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.

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