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A US-led coalition fighting the Islamic State group in Syria and Iraq has admitted to killing 40 civilians in an airstrike on a school sheltering displaced civilians near IS former Syrian stronghold of Raqqa.

The Combined Joint Task Force (CJTF) made the announcement in a on Thursday, after human rights groups had urged the force to probe the deadly strike in the town of al-Mansoura last March.

"During a strike on Daesh militant multifunctional centre allegedly caused civilian casualties. Forty civilians were unintentionally killed," the statement said, using the Arabic acronym for IS.

"The report was reopened after the receipt of new evidence from Human Rights Watch," it added.

The CJTF had initially denied any civilians had died in the attack and said reports of civilian casualties due to coalition strikes were "vastly inflated".

HRW said in a report last September that the coalition air raid at Badia school killed at least 40 people, including 16 children. The UN placed the civilian toll in the attack at 150 or more deaths.

The report also said the coalition killed at least 44 people, including 14 children in a strike two days later on a crowded market and bakery in the city of Tabqa.

The coalition failed to comment on the marketplace bombing its latest statement.

The bombings highlight the high-cost of the US air war on IS and its strikes on densely-packed civilian areas.

Amnesty International said this month that the US-led military campaign to oust IS from Raqqa in 2017 killed hundreds of civilians in indiscriminate bombing, amounting to possible war crimes.

The military operation failed to take "adequate account" of civilians and the "precautions necessary to minimise harm" to them in the city, IS' de facto capital in Syria, the human rights group said in a report.

"Coalition claims that its precision air campaign allowed it to bomb IS out of Raqqa while causing very few civilian casualties do not stand up to scrutiny," the report concluded.

Last year, the coalition admitted it "probably played a role" in the deaths of up to 150 people in an airstrike on the Iraqi city of Mosul.