BEIRUT, Lebanon (AFP) — A US-led coalition air strike killed 21 civilians on Monday as they tried to escape the Islamic State group’s Syrian bastion of Raqa, the Syrian Observatory for Human Rights said.

“The civilians were boarding small boats on the northern bank of the Euphrates River to flee southern neighbourhoods of Raqa,” said Observatory head Rami Abdel Rahman.He said women and children were among the dead but he could not immediately give a specific number.Thousands of civilians have fled the northern city as an offensive by the Syrian Democratic Forces (SDF), an Arab-Kurdish alliance, closes in.The SDF assault has been backed by air strikes from the Washington-led coalition bombing IS in Iraq and Syria since 2014.Russian aircraft have also carried out bombing raids against IS convoys fleeing the city.According to the Britain-based Observatory, the latest coalition raids took place on Monday morning.”The toll may continue to rise as some of the wounded are in critical condition,” the monitor said.The US military has said coalition air strikes in Iraq and Syria had “unintentionally” killed 484 civilians, but observers say the number is much higher.Syria’s conflict erupted in March 2011 with widespread protests against the regime, but it has since morphed into a multi-front war that has left 320,000 people dead.