More than 170 civilians have been killed by U.S.-led strikes on IS 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 U.S.-led coalition against IS 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 in strikes that destroyed buildings where families were sheltering.

The Britain-based Observatory said that was the single largest daily death toll since the U.S.-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 IS inside the city.

Former Raqqa residents in touch with relatives still in the city echoed this view to Reuters.

The U.S.-led coalition says it is careful to avoid civilian casualties in its bombing runs against IS 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 multi-storey 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 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 U.S.-backed forces close in on IS.

Footage of the Old City released by activists showed extensive damage to buildings around the historic Old Mosque.

Amaq, an IS-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" 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.