BEIRUT — Dozens of civilians have died in two days of intense US-led strikes on Raqqa, a monitor said Tuesday, as fighting to retake the Syrian city from militants nears its densely populated center.

The US-backed Syrian Democratic Forces (SDF) have captured just under 60 percent of Raqqa, monitors say, leaving militants from Daesh (the so-called IS) group in control of about 10 sq. km in the heart of the city.

But as clashes approach central Raqqa, monitors and activists have reported scores killed in intensifying coalition bombardment of the city.

On Monday, US-led airstrikes killed at least 42 civilians in several neighborhoods in Raqa under Daesh control, according to the Britain-based Syrian Observatory for Human Rights monitor.

Nineteen children and 12 women were among the dead.

That takes to 167 the number of civilians killed in coalition strikes since August 14, after the observatory said at least 27 were killed on Sunday.

"The tolls are high because the air strikes are hitting neighborhoods in the city center that are densely packed with civilians," observatory director Rami Abdel Rahman said.

"There are buildings full of civilians who are trying to get away from the front lines."

The SDF's Arab and Kurdish fighters broke into Raqqa in early June after spending months chipping away at Daesh-held territory in the surrounding province.

The US-led coalition, which operates in both Syria and neighboring Iraq, says it takes all possible measures to avoid civilian casualties.

Earlier this month it acknowledged the deaths of 624 civilians in its strikes in Syria and Iraq since 2014, but rights groups say the number is much higher.

Asked about the escalating civilian tolls in recent days, SDF spokesman Talal Sello told AFP his forces were striving to avoid casualties.

"One of the major reasons for the slow progress in the Raqqa fight is the preserving of civilian lives and avoiding massive losses among them," Sello said.

He blamed Daesh for using civilians as "human shields."

"We have opened up safe routes for civilians to cross securely toward areas controlled by our forces, who are rescuing civilians almost daily and transferring them to safe places."

Tens of thousands of people have fled Raqa city, with the United Nations estimating up to 25,000 civilians remained trapped inside with dwindling food and fuel supplies.

The UN's humanitarian pointman for Syria, Jan Egeland, has said Daesh-held territory in Raqqa city is now "the worst place" in the war-torn country.

Civilians, including women and children, must dodge sniper fire, Daesh-laid mines, and coalition bombardment to make it out of the city alive.

Activist collective Raqqa is Being Slaughtered Silently (RBSS) also reported heavy raids in recent days.

It said those killed since Sunday included entire families and people displaced to Raqqa from other parts of Syria.

RBSS published gruesome photos of dust-covered toddlers who had reportedly died in the bombardment.

More than 330,000 people have been killed since Syria's conflict erupted in March 2011 with protests against President Bashar Al-Assad. — AFP