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The deaths came after 12 women were killed in a strike by the US-led coalition fighting IS in the east of Syria's Raqqa province on Sunday, the Syrian Observatory for Human Right said.

Monday's strike hit the town of Albu Kamal in the early hours of the morning, Observatory director Rami Abdel Rahman said.

"They hit a residential area at 3:00 am while people were sleeping, causing the high toll," he said.

He said at least 30 civilians including 11 children and six women were killed, and that the toll could rise because of people trapped under debris.

He added that IS was using some apartments in the area targeted as local headquarters.

The deaths came after 12 women were killed in a US-led coalition strike on Sunday.

The Observatory said that strike hit vehicles carrying farmworkers home from fields in the afternoon.

IS has lost swathes of the territory it once held in Raqqa province, though it still holds the city and some areas to the east.

A US-backed alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces is battling towards Raqqa city, the extremist group's most important remaining Syrian bastion.

The US military said in May that coalition strikes in Syria and Iraq had "unintentionally" killed 352 civilians since it launched operations against IS in 2014.

Rights groups say the actual figure is much higher.