AMMAN/BEIRUT (REUTERS, AFP) - More than 170 civilians have been killed by US-led strikes on ISIS in Raqqa city in the past week, a spike in casualties since an offensive to oust the militants began more than two months ago, a war monitor and sources said.

The US-led coalition against the Islamic State in Iraq and Syria (ISIS) said attacks on militant targets were conducted routinely and the allegation had been sent to their teams for assessment. The monitoring group Syrian Observatory for Human Rights said at least 42 people, including 19 children and 12 women, were killed on Monday (Aug 21) in strikes that destroyed buildings where families were sheltering.

The Britain-based Observatory said that was the single largest daily death toll since the US-backed Syria Democratic Forces (SDF), a group of Kurdish and Arab militias, began their assault on Raqqa last June after a long campaign to isolate ISIS inside the city. Former Raqqa residents in touch with relatives still in the city echoed this view to Reuters.

The US-led coalition said it is careful to avoid civilian casualties in its bombing runs against ISIS in both Syria and Iraq, and investigates any allegations.

"The coalition respects human life, and is assisting partner forces in their effort to liberate their land from ISIS while safeguarding civilians. Our goal is always for zero civilian casualties," a statement sent to Reuters by the Pentagon said.

The Observatory said bombs that hit the al-Sakhani and Harat al-Bado districts on Monday were close to a multistorey residential building that had been bombed the day before, killing at least 27 civilians, including seven children.

Former Raqqa residents in touch with relatives gave a higher death toll, saying many bodies were still under the rubble.

The SDF alliance, spearheaded by the Kurdish YPG militia, has been waging fierce battles inside the Old City area of Raqqa since last month. Fighting is now intensifying near the city centre as the US-backed forces close in on ISIS.

Footage of the Old City released by activists showed extensive damage to buildings around the historic Old Mosque. Amaq, an ISIS-affiliated news agency, released a video on Monday showing at least a dozen corpses, many of them children, lying on the floor. It claimed the video was shot in Raqqa on Monday and showed victims of coalition air strikes, as well as extensive damage to residential areas.

Reuters could not verify the authenticity of the video.

An activist-run group Raqqa Is Being Slaughtered Silently (RBSS) said it had documented at least 946 civilian deaths since the Raqqa offensive began in June. The United Nations says at least 200,000 people have fled Raqqa in recent months and that up to 20,000 civilians remain trapped inside.

The plight of civilians left in the city has worsened, with water cut off for over two months and shortages of food, leaving many of those remaining living on canned foods.

'Buildings full of civilians'

"There are buildings full of civilians who are trying to get away from the front lines," Observatory's director Rami Abdel Rahman told the AFP news agency.

The coalition says avoiding civilian casualties is its "highest priority". Coalition spokesman Ryan Dillon told AFP that the latest allegations of civilian deaths would be taken seriously and investigated. He said the coalition had stepped up its strikes in Raqqa since a US-backed offensive successfully ousted ISIS from Mosul in neighbouring Iraq, freeing up aircraft.

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

The SDF's Arab and Kurdish fighters broke into Raqqa in early June after spending months chipping away at ISIS-held territory in the surrounding province. Asked about the escalating civilian toll 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," Mr Sello said. He blamed ISIS 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."

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

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

Extremists' days 'numbered'

RBSS also reported heavy raids in recent days, and said those killed since Sunday included entire families and people displaced to Raqqa from other parts of Syria.

"Unfortunately, civilians have no way to protect themselves," said RBSS' 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 ISIS seized Raqqa in 2014, the city became synonymous with the group's most brutal practices, including beheadings and public executions.

It served as the extremist faction's de facto capital in Syria, alongside its twin pivot Mosul in Iraq. After ousting ISIS from Mosul, Iraqi forces are now battling the extremists in Tal Afar, the group's last major bastion in northern Iraq.

US Defence Secretary James Mattis arrived in Baghdad on Tuesday for talks with top Iraqi officials on keeping the pressure up on ISIS. "ISIS' days are certainly numbered, but it's not over yet and it's not going to be over anytime soon," he said.