AIN ISSA, Syria/BEIRUT/AMMAN: Daesh militants have become increasingly desperate in the face of heavy airstrikes on their Syrian bastion Raqqa, passing themselves off as civilians to escape detection and killing anyone who tries to flee, witnesses said.

At a camp for the displaced in the village of Ain Issa north of the city, people who arrived Wednesday also said the airstrikes supporting an assault by U.S.-backed forces had inflicted widespread destruction as the battle intensified.

United Nations war crimes investigators said the air campaign had killed at least 300 civilians in the city, captured by Daesh in 2014 in the chaos of Syria’s civil war.

The escapees said the airstrikes had flattened rows of apartment blocks along a main road but many of them had already been abandoned by residents fleeing Daesh’s reign of terror and the assault on the town, which began last week.

“The coalition strikes destroyed a four-story apartment building. I saw 10 people trapped underneath,” said Abu Hamoud. “They used phosphorus.”

Human Rights Watch expressed concern Wednesday about the use of incendiary white phosphorous weapons by the U.S.-led coalition, saying it endangers civilians when used in populated areas.

The coalition is backing the Syrian Democratic Forces (SDF), a group of Kurdish and Arab militias who have been closing in on Raqqa in northern Syria for months in preparation for the assault to recapture the city.

Hasan Kirfou said an airstrike hit the mosque where he works just a few hours after he closed it for the night, and two other mosques were hit.“I saw three dead teenagers on top of each other outside the Nour mosque,” he said. “I don’t know why they shot these areas. As far as I know there were only a few Daesh snipers left there.”

Much of the damage from airstrikes was inflicted on Seif al-Dawla street, one of the main arteries of the town. “My uncle and two cousins were killed. Their house was destroyed,” Kirfou said.

The pressure on Daesh, which is on the brink of losing the other center of its self-proclaimed caliphate, the Iraqi city of Mosul, is taking a heavy toll on the group, people who arrived at Ain Issa in the last few days said.

“They have started using microphones to tell people: ‘Don’t go to the infidels, stay with Islam,’” said Abdul Razak Crais, standing near rows of white tents as people lined up for food.

“They poured gasoline on the cars of anyone who tried to escape then lit a match and burned the vehicles. I saw them haul people out of their cars and shoot them with AK-47s.”

The coalition estimates that 3,000-4,000 Daesh fighters are holed up in Raqqa, the group’s administrative headquarters.

In other developments, a U.N. investigative commission for Syria said that last month’s “de-escalation” agreement has reduced violence in only one of the four zones included in the deal and has not led to greater humanitarian access to besieged areas across the war-torn country.

Underscoring the ongoing violence, a barrage of airstrikes and artillery shells pummeled different areas of southern Deraa province Wednesday, leaving at least eight people dead, including children, first responders and activists said.

In Geneva, the Independent International Commission of Inquiry told the U.N. Human Rights Council that fighting around the central province of Homs, near Damascus and in the southern city of Deraa has continued – despite the deal brokered by Russia, Turkey and Iran in May.

Only in the northern Idlib province and western Aleppo has there been “discernible reduction” in violence, said the chair of the U.N. commission, Paulo Sergio Pinheiro.

Violence in Deraa has escalated, with some of the most intense bombing reported in recent weeks.

Activists and rescue workers Wednesday reported at least 12 airstrikes and artillery shelling in different areas of Deraa, the southern province divided between insurgents and government forces.

Syrian Civil Defense first responders, also known as White Helmets, reported airstrikes on a school that served as a shelter for displaced people in Tafs, in the province’s west.

Initial reports said at least eight people from the same family were killed, according to the first responders who said the displaced had gathered at the site of the aftermath of an earlier strike nearby.

The Syrian Observatory for Human Rights put the death toll at nine. A video from first responders distributed on social media shows bodies strewn on the floor as rescuers search for survivors, lifting children out of the rubble amid wails from women.

Separately, U.N. envoy to Syria Staffan de Mistura told reporters during at a conference in Norway that fresh negotiations aimed at ending the war could start at the end of the holy Muslim month of Ramadan.

Meanwhile in Syria’s southeastern desert, U.S. troops based have expanded their footprint, rebels there say, increasing the risk of direct ground confrontation between the Americans and Iran-backed pro-government forces.

Abu al-Atheer, military spokesman for the U.S.-backed Maghawir al-Thawra rebel group, told Reuters U.S. forces had spread from their initial location at Tanf to set up a second base at Zakf, around 60-70 km to the northeast.