BEIRUT: A US-led coalition air strike killed at least 18 civilians on Tuesday in Daesh's former stronghold of Raqqa, the Syrian Observatory for Human Rights monitor said.

"International coalition planes targeted water wells where a group of civilians were gathered in the north of Raqqa city, killing at least 18 civilians," Observatory director Rami Abdel Rahman said.

The Britain-based group said four children were among the dead.

The US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, broke into Raqqa in June and has since wrested 90 per cent of the city from Daesh.

But activists say coalition strikes in support of the operation have killed hundreds of civilians and caused enormous damage.

The coalition's spokesman Colonel Ryan Dillon said it does "everything within its power to limit harm to non-combatants and civilian infrastructure."

He said "rigorous standards" were applied to coalition targeting and "extraordinary efforts" taken to avoid civilian deaths.

"The coalition conducts a detailed assessment of each and every allegation of possible civilian casualties and we will do so for this allegation as well," he added in an email to AFP.

In late September, the coalition acknowledged the deaths of 735 civilians in its strikes on Syria and Iraq since 2014.

But activists say the toll is much higher.

Raqqa has faced water shortages for months because of damage to pipelines caused by suspected coalition strikes.

Even in the early days of the Raqqa assault, residents said they feared being caught in air strikes or shelling when they ventured to wells or the Euphrates River that runs south of the city to draw water.