US-led coalition strikes on Sunday killed 27 civilians in part of Syria's Raqqa city held by the Islamic State group, the Syrian Observatory for Human Rights monitor said.

Seven children were among the dead in the strikes that "hit the densely-populated Al-Badu area in the center of the city," Observatory director Rami Abdel Rahman said on Monday.

Once a jihadist stronghold, more than half of Raqqa city has fallen to the Syrian Democratic Forces, an alliance of Kurdish and Arab fighters that entered the city in June.

The SDF is heavily backed by the US-led coalition, which has regularly launched air strikes against the jihadists that have reportedly killed scores of civilians.

According to the Britain-based Observatory, at least 125 civilians have been killed in a week of US-led strikes on Raqqa city, including those who died on Sunday.

"There are civilians killed each day in coalition strikes... The closer the fighting gets to the densely-populated city center, the more civilian deaths there are," Abdel Rahman said.

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

In August, 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.

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

The UN estimates there are up to 25,000 civilians trapped inside the city, with food and fuel supplies short and prohibitively expensive.

Tens of thousands of civilians have also fled the city, risking IS sniper fire and landmines in the process.

More than 330,000 people have been killed in Syria since the conflict erupted in March 2011 with anti-government protests.

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