SIRTE, Libya, July 21 (Reuters) - Forces aligned with Libya's U.N.-backed government fought fierce exchanges with Islamic State militants in Sirte on Thursday in which 13 of their fighters were killed and more than 110 wounded, officials said.

The government-backed forces have pinned militants back to a small area in the centre of the coastal city after advancing on the Islamic State stronghold in May.

But they have faced stiff resistance from the several hundred militants thought to be still holed up in Sirte, suffering casualties from snipers and explosive devices.

On Thursday the government-backed forces said in statements that they were advancing on two fronts after bombarding Islamic State positions from the air and the ground at dawn. They said there was heavy fighting in the Dollar neighbourhood and around the port.

The forces are mainly made up of fighters from the city of Misrata, though other western Libyan towns and cities have also contributed men.

They have received some support from the U.N.-backed Government of National Accord (GNA) in Tripoli, but many fighters feel let down by the government and its Western allies, saying that a lack of medical support, equipment and ammunition has hindered their progress.

The frontline had been relatively calm earlier this week as the brigades regrouped after suffering heavy losses last Friday.

Islamic State took total control of Sirte last year, turning it into its primary North African base and extending its presence along about 250 km (155 miles) of Libya's coastline.

The militant group 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.

(Reporting by Goran Tomasevic and Ahmed Elumami; Writing by Aidan Lewis; Editing by Angus MacSwan)

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