A suspected U.S.-led coalition strike in Syria's Raqqa province has killed 12 women, the Syrian Observatory for Human Rights said Monday.

The Britain-based monitor said the strike Sunday afternoon hit vehicles carrying farmworkers home from fields in the east of the province.

The monitor said it believed the strike had been carried out by the U.S.-led coalition fighting Daesh terrorist group in Syria and Iraq.

The Observatory relies on a network of sources inside Syria and says it determines whose planes carry out raids according to their types, locations, flight patterns and the munitions used.

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

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

Rights groups say the actual figure is much higher.