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 approaches its densely populated centre.

The coalition acknowledges it has pounded the city and surrounding area with more than 250 air strikes over the past week alone, in support of the Syrian Democratic Forces alliance battling the Islamic State group.

The SDF has so far captured just under 60 percent of Raqa, monitors say, leaving IS with 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.

On Monday, US-led air strikes killed at least 42 civilians in several neighbourhoods of Raqa under IS control, according to the Britain-based Syrian Observatory for Human Rights.

Nineteen children and 12 women were among the dead, the monitor said.

The Observatory said 167 civilians have been killed in coalition strikes since August 14, including 27 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.

- 'Buildings full of civilians' -

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

The coalition says avoiding civilian casualties is its "highest priority".

Speaking in Baghdad, US Defence Secretary Jim Mattis insisted IS tactics were to blame for placing civilians in danger.

"We are the good guys and the innocent people on the battlefield know the difference," he said.

But the coalition acknowledges it has escalated its strikes on Raqa, with more aircraft available since a US-backed operation successfully pushed IS from Mosul in neighbouring Iraq last month.

Coalition spokesman Colonel Ryan Dillon said more than 250 strikes had hit Raqa and its surroundings in the past week. An AFP count of the coalition's own reporting put the figure at over 300 strikes.

"It's probably logical to assume there has been some increase in civilian casualties. But I would ask someone to show me hard information," coalition commander Lieutenant General Stephen Townsend said.

Dillon told AFP the latest allegations of civilian deaths would be taken seriously and investigated.

The coalition, which operates in both Syria and Iraq, earlier this month acknowledged the deaths of 624 civilians in its strikes in the two countries since 2014, but rights groups say the real toll is much higher.

The UN's Office for the Coordination of Humanitarian Affairs on Tuesday expressed "deep concern" and said all parties should "abide by their obligations" to protect civilians.

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.

SDF spokesman Talal Sello told AFP that efforts to avoid "massive losses" of civilian life were "one of the major reasons for the slow progress in the Raqa fight".

"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," he said.

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

Civilians, including women and children, must dodge sniper fire, IS-laid mines, and coalition bombardment to escape the city.

- Jihadists' days 'numbered' -

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

"Unfortunately, civilians have no way to protect themselves," said the group's Husaam Essa.

"All they can do is try to hide in whatever shelter they can and avoid going out into the street as much as possible," he told AFP.

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 Iraq.

After ousting IS from Mosul, Iraqi forces are now battling the jihadists in Tal Afar, the group's last major bastion in northern Iraq.

Mattis in Baghdad on Tuesday met top Iraqi officials for talks on the next phase of the fight.

The jihadist group's "days are certainly numbered, but it's not over yet and it's not going to be over anytime soon", he said.

Source: AFP