RAQQA, Syria (Reuters) - In the gutted four-storey building that he and five other U.S.-backed fighters have turned into a frontline fortress, Babel can peer out at Islamic State positions just 150 meters away.

A British volunteer fighter of Syrian Democratic Forces runs for cover to avoid sniper fire of Islamic State militants, at the frontline in Raqqa, Syria. REUTERS/Erik De Castro

The jihadist militants expected to make a last stand for this stronghold of their self-proclaimed caliphate are cornered and desperate here in Raqqa, the city on the Euphrates river that has served as their de-facto Syrian capital since 2014.

“Let Daesh come - we’re ready for them. We have explosives to drop downstairs,” said Babel, which is his nom de guerre. He and his fellow members of the U.S.-backed Syrian Defense Forces have occupied the frontline building for three weeks and are now preparing for a final showdown with Islamic State, the group also known as ISIS, ISIL or Daesh.

The SDF fighters’ position is well prepared in case the cornered IS militants attempt an attack. The front porch steps have been demolished, leaving a drop to the basement that can be crossed only by climbing a section of iron fence - a makeshift ladder, which the defenders can pull up. Inside, plastic bottles have been strewn across the floor to squeak when stepped on and alert them to intruders.

After months of intense fighting and heavy U.S. bombardment, the SDF have surrounded Islamic State militants in a small part of the city. As the Kurdish and Arab militias of the SDF close in and U.S.-led air raids increase, they expect fierce fighting to mark the final stages of the campaign.

“Daesh regularly launch small raids, even behind our position. Yesterday they attacked the building opposite and tried to push toward us. We killed a few and they retreated to the hospital,” Babel said.

SDF units have a clear view over the Raqqa hospital, one of Islamic State’s last strongholds in the city, from a line of buildings they occupy to its northwest. Apartment blocks between them and the hospital have been flattened by air strikes.

Commanders say the hospital and a nearby stadium, where the jihadists are said to be holding civilian hostages, will be where they make their last stand.

Babel’s unit are holding the front line ahead of an anticipated final push, firing at militants whenever they can spot them.

“The last few nights they’ve been shining spotlights from the hospital towards our lines, so we can’t really see,” he said.

Another fighter in the unit said sniper fire from Islamic State had recently reduced, possibly as a means of conserving ammunition for more intense fighting to come.

SURRENDER OR DIE

An SDF field commander said on Sunday assaults were to begin soon as part of a final push against Islamic State, focused initially on surrounding the stadium. The hospital is already encircled.

“Daesh is amassing, preparing to fight. This is the last stage, so they’ll resist and then surrender or die,” the commander who gave his name as Ardal Raqqa said.

U.S. coalition spokesman Ryan Dillon was more cautious about calling the current stage of the offensive a final assault.

“Whether this is the final assault or an assault, I won’t characterize it either way,” Dillon said by phone. The fight was “concentrated around the complex which was the national hospital and the stadium,” he said.

The SDF predicted ahead of a major push in June that it could take just weeks to drive Islamic State out of Raqqa. That has proven overly optimistic, with the militants holding out for months so far.

Their use of civilians for cover, tunnels to launch counter attacks, snipers and countless booby traps have slowed SDF advances.

“Many are being wounded, especially by snipers. Daesh snipers often shoot to wound, not to kill, so they can target whoever comes to save an injured comrade,” Babel said.

One shaken SDF fighter in a nearby base last week showed cuts on his face from where a sniper bullet had ricocheted.

Both the U.S. spokesman Dillon and the SDF fighter Babel said that some Islamic State fighters were surrendering.

“The other day a Saudi fighter escaped and handed himself over. The guys who surrender usually have families,” Babel said.

Interrogations of surrendering militants revealed IS had dug a tunnel between the hospital and stadium, he said.

Dillon said several IS militants and leaders had surrendered in recent weeks. “It is a growing trend,” he said.

At the front line, SDF fighters said morale was high. One fighter belted pop music to other units over his walkie talkie.

“Hopefully we’ll be done soon,” Babel said.