TRIPOLI (Reuters) - Forces aligned with Libya’s unity government battled Islamic State on Thursday in the militant group’s stronghold of Sirte, but faced resistance from snipers as they edged towards the city center.

Brigades mainly composed of fighters from the western city of Misrata have advanced rapidly, driving the militants back along the coastal road west of Sirte before seizing strategic points on the edge of the city.

A separate militia that controls terminals in Libya’s oil crescent, the Petroleum Facilities Guard (PFG), said it had advanced further from the east to reach the town of Harawa, about 70 km (44 miles) east of Sirte.

If the advances are sustained, they could dislodge IS from its most important base outside the Middle East and provide a boost to the U.N.-backed Government of National Accord (GNA).

Mohamed al-Gasri, a military spokesman based in Misrata, said fighting was underway on Thursday near the Ouagadougou conference hall, where IS holds religious instruction sessions.

“We think that Sirte will be liberated within days, not weeks,” Gasri said. “The Daesh (Islamic State) snipers are a concern to us because they shoot from long distances and that has hindered us in the battle inside the city.”

The media office for the brigades said in a statement that Libyan air force jets had carried out strikes near the Ouagadougou hall.

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The brigades had already claimed control over a number of strategic sites on Sirte’s outskirts including an air base, several military camps and a roundabout where IS had previously hung the bodies of executed enemies.

Dozens of brigade members have been killed and hundreds wounded in the past month of fighting. On Wednesday alone, 15 men were killed and 95 injured, a Misrata hospital spokesman said.

The main hospital in Misrata is overflowing and some fighters have been flown to Turkey or Italy for treatment. On Thursday the GNA appealed in a statement for further international medical aid “for our heroes at the front lines”.

SETBACKS

Islamic State established a presence in several Libyan cities from late 2014, taking full control of Sirte, hometown of veteran ruler Muammar Gaddafi, the following year. It also seized about 250 km (155 miles) of Mediterranean coastline either side of Sirte.

But the group has struggled to win support or retain territory elsewhere in Libya, suffering recent setbacks in both the east and west of the country.

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The GNA is designed to replace two rival governments that have competed for power from Tripoli and from the east since 2014, backed by complex alliances of armed groups.

Both the PFG and key armed groups from Misrata have pledged to support it. Western powers see the new government as the best chance of ending the turmoil plaguing Libya since Gaddafi was forced from power in an uprising five years ago.

At the Pentagon, spokesman Peter Cook said the United States was encouraged by the progress on the ground but said no decisions had been made regarding greater U.S. military involvement in Libya, where teams of U.S. special operations forces have been rotating in and out of the country for months to get to know local fighters on the ground.

“We haven’t made any additional decisions about U.S. action at this point. We’re obviously watching it very closely and (are) very encouraged by what we see,” Cook told a news briefing.

BENGHAZI VIOLENCE

Since arriving in Tripoli in March the GNA has sought to meld some of Libya’s key armed factions into a unified security force, even as it continues to face resistance from political and military hardliners in the east.

These include eastern military commander Khalifa Haftar, who has been conducting a campaign against Islamists and other opponents in Benghazi for the past two years. Heavy fighting and air strikes have continued there over the past two days, with at least eight of Haftar’s forces killed, medical and security officials said.

Eastern forces have also been engaged in fighting in the eastern city of Derna, where rival Islamists forced Islamic State to retreat. On Thursday, a resident told Reuters that an air strike had hit a residential area, killing a woman and three children.

The GNA appointed another eastern commander, Mahdi al-Barghathi, as minister of defense. He has been trying to peel away support from Haftar, and last week two military units in Benghazi announced their support for the GNA.

For now the common cause of defeating Islamic State and diminishing Haftar’s influence has driven Misrata, Barghathi and the PFG to cooperate, said Mattia Toaldo, an analyst at the European Council for Foreign Relations.

“For Libyan standards it’s quite a remarkable degree of coordination, and it’s coordination between people who have been fighting each other until a year ago.”

Any recent gains for the GNA are fragile, however, with Haftar bound to attempt a comeback and current allegiances at risk of dissolving if Sirte is taken, Toaldo said.