MISRATA, Libya (Reuters) - Forces loyal to Libya’s U.N.-backed unity government say they aim to encircle Sirte, Islamic State’s stronghold in Libya, having moved to within 15 km (10 miles) of the city center.

The forces, composed of fighters mostly from the western city of Misrata, are now closer to Sirte than they have been for nearly a year. Last summer Misrata brigades withdrew from Sirte and Islamic State took full control there.

Western states are hoping the unity government can bring together Libya’s competing factions to defeat Islamic State, though the new government has struggled to secure support beyond its power base in the west of the country.

Earlier this month Islamic State surged forward towards Misrata, which lies about 240 km northwest of Sirte, taking control of the town of Abu Grain and a number of villages and checkpoints in the area before being pushed back.

Military spokesman Mohamed al-Gasri said that after advancing along the road west of Sirte on Friday, government-backed forces were seeking full control of a steam plant about 15 km from central Sirte, as well as a road leading south from Sirte to Waddan.

“The next step is to encircle Sirte, and then we will ask the residents to try to leave,” he said. “We don’t want to enter now because of the residents. But if it becomes a battlefield we can enter within hours.”

Misrata brigades have suffered some of their heaviest losses for months in recent clashes. One single truck bombing killed 32 people last week, and some 75 fighters have been killed and more than 350 injured since the start of May, Gasri said.

He also said that dozens of Islamic State fighters had died, and the Misrata military operations room said on Saturday that these included a senior North Africa commander for the group called Khaled al-Shayab.

On the road south of Misrata burnt-out cars can be seen at the sites of more than half a dozen suicide bombings or mine explosions, and the government-backed forces are still struggling to de-mine areas where Islamic State advanced.

For now, the most visible forces on the government-backed side are young fighters with mismatched uniforms, pick-up trucks - resembling the rebel forces that fought in the NATO-backed campaign to topple Muammar Gaddafi five years ago.

About 50 km from Sirte troops are building earth and sand barriers on the coastal road as a defense against further bombings.

Gasri said last year’s withdrawal from Sirte would not be repeated.

“This time it’s different because there is an internationally recognized government that has pledged to support the army to fight Daesh (Islamic State).”

Though U.S. and European special forces are known to have been on the ground in Libya, Gasri said his forces had not received any direct international assistance.

Forces in eastern Libya that have clashed with Misrata in the past have announced a separate campaign to capture Sirte, leading to fears of renewed internal conflict.

The eastern forces are allied to a parliament and government based in the east that failed to formally endorse the U.N.-backed administration.