A US-led coalition air strike killed at least 18 civilians on Tuesday in the Islamic State group's former stronghold of Raqa, 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 Raqa 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 Raqa in June and has since wrested 90 per cent of the city from IS.
But activists say coalition strikes in support of the operation have killed hundreds of civilians and caused enormous damage.
The coalition says it takes "extraordinary care" to avoid civilian casualties in its strikes and that it investigates credible claims of civilian deaths.
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.
Raqa has faced water shortages for months because of damage to pipelines caused by suspected coalition strikes.
Even in the early days of the Raqa 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.
