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At least 42 civilians were killed Monday in a barrage of US-led air strikes on Islamic State group territory in the Syrian city of Raqqa, a monitor said.

Nineteen children and 12 women were among those killed in the raids, which hit several neighbourhoods in the northern city, the Syrian Observatory for Human Rights told AFP on Tuesday.

The toll marked the second consecutive day of ferocious bombardment on Raqqa, more than half of which has been captured by the US-backed Syrian Democratic Forces battling IS.

The latest figures for Monday's raids take 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," said Observatory director Rami Abdel Rahman.

"There are buildings full of civilians that are trying to get away from the front lines.

"Coalition air strikes are targeting any building where any kind of Daesh movements are being detected," he told AFP, using an Arabic acronym for IS.

The US-led coalition, which operates in both Syria and neighbouring 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 United Nations estimates there are up to 25,000 civilians trapped inside the city, with food and fuel supplies short and prohibitively expensive.

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

Peace plan

The next talks to try to thrash out a Syria peace plan in Kazakhstan's capital Astana may be held in mid-September, after Russia had planned to hold them in late August, it was announced on Tuesday.

Kazakhstan's foreign ministry wrote on Facebook, quoting diplomatic chief Kairat Abdrakhmanov, that the timing of the talks would be set at a meeting this month between experts from Russia, Turkey and Iran and "provisionally, we could be talking about mid-September".

Abdrakhmanov said that the date change was based on "information received from Russia".

Earlier peace talks in Astana saw Russia, Turkey and Iran hammer out a plan to establish "de-escalation" zones across swathes of Syria.

More than 400,000 people have been killed in Syria's conflict, which began in 2011 after President Bashar al-Assad's regime responded with military force to peaceful protests demanding democratic reforms. It has since mushroomed into all-out war drawing in world powers and displacing millions of Syrians.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.