Islamic State militants have been targeted by US air strikes (AP)

The Islamic State has ordered an evacuation from its de facto capital Raqqa following warnings that a dam could collapse, Syrian activists said.

The activist-run Raqqa Is Being Slaughtered Silently group said civilians are fleeing the town en masse.

It said the militants are warning residents that the Taqba dam upstream on the Euphrates River is out of service, weakened by US-led coalition air strikes, and could collapse. Water levels are said to be rising behind the dam.

The Britain-based Syrian Observatory for Human Rights, an opposition-run monitoring group, is also reporting the dam is out of service.

The two groups rely on local contacts to smuggle information out of IS-held territory.

US-backed Syrian Kurdish forces are battling IS near the militant-held dam as part of a broader campaign to take Raqqa.

Meanwhile, a leading Syrian opposition group called on the US-led coalition to stop targeting residential areas in and around Raqqa.

The Syrian National Coalition (SNC) said it is "increasingly concerned" about civilian casualties in the campaign against the extremist group. The exiled opposition coalition is taking part in UN-mediated talks in Geneva.

The SNC said it believes coalition forces were behind an air strike that killed at least 30 civilians sheltering in a school in the countryside outside Raqqa on March 21. The coalition has said it is investigating.

The US has provided substantial air and ground support to the Kurdish-led Syrian Democratic Forces, who are closing in on Raqqa.

AP