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

The US-backed Syrian Democratic Forces (SDF) have captured just under 60 percent of Raqa, monitors say, leaving jihadists from the Islamic State group in control of about 10 square kilometres (four square miles) in the heart of the city.

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

On Monday, US-led air strikes killed at least 42 civilians in several neighbourhoods in Raqa under IS 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 neighbourhoods in the city centre 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 Raqa in early June after spending months chipping away at IS-held territory in the surrounding province. 

The US-led coalition, which operates in both Syria and neighbouring 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.

- 'Safe routes' -

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 Raqa fight is the preserving of civilian lives and avoiding massive losses among them," Sello said. 

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

"We have opened up safe routes for civilians to cross securely towards 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 IS-held territory in Raqa city is now "the worst place" in the war-torn country.

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

Activist collective Raqa 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 Raqa 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. 

After IS seized Raqa in 2014, the city became synonymous with the group's most brutal practices, including beheadings and public executions. 

It served as the jihadist faction's de facto capital in Syria, alongside its twin pivot Mosul in neighbouring Iraq.

A US-backed offensive ousted IS from Mosul in July and is now chasing the militants in Tal Afar, the group's last major bastion in northern Iraq. 

US Defence Secretary Jim Mattis arrived in Baghdad on Tuesday for talks with top Iraqi officials on keeping the pressure up on IS. 

"ISIS's days are certainly numbered, but it's not over yet and it's not going to be over anytime soon," he said, using an alternative acronym for IS.

Source: AFP