Syrian activists say the Daesh group has ordered an evacuation from its de facto capital, Raqqa, following warnings that the Tabqa Dam upstream on the Euphrates River could collapse.

The activist-run Raqqa is Being Slaughtered Silently says civilians are fleeing Raqqa en masse. It says the militants are warning residents that the dam is out of service, weakened by U.S.-led coalition airstrikes, and could collapse. It says water levels are 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 Daesh-held territory.

The U.S.-backed Syrian Democratic Forces (SDF) are battling Daesh near the militant-held dam as part of a broader campaign to take Raqqa.

A technical source also confirmed AFP that the dam is out of service as of Sunday as the fighting damaged its power station, risking dangerous rising water levels.

"Shelling on the area... that supplies that dam with electricity has put it out of service," the source said.

"The work needed to fix the problem is not possible because there is not sufficient staff available as a result of the intensive shelling in the area of the dam," he added.

"If the problem is not fixed, it will begin to pose a danger to the dam."

The source could not confirm what kind of shelling damaged the power station, but there has been heavy fighting nearby as well as air raids by the U.S.-led coalition against Daesh in support of SDF fighters in the area.

SDF spokesman Talal Sello insisted there was no imminent danger to the dam, which is Syria's largest and sits on the Euphrates River.

"There have been no air strikes on the dam," he told AFP.

SDF forces were airdropped behind Daesh lines last week by U.S. forces to begin their assault on the dam, which is around 35 miles (55 kilometers) west of Raqqa.

"We carried out this operation to land there to avoid shelling or damage to the dam," Sello said.

SDF fighters reached one of the entrances of the dam on Friday, battling Daesh in clashes in which militants had been killed and wounded.

But the dam remains under Daesh control, with SDF progress being hampered by the exposed nature of the terrain, which is also heavily mined, the Syrian Observatory for Human Rights monitor said.

Daesh issued warnings through its propaganda agency Amaq warning the dam "is threatened with collapse at any moment because of American strikes and a large rise in water levels."

But the source at the dam told AFP there had not yet been significant water level increases, though he acknowledged levels would rise if the facility remained out of service.

Earlier this month, the UN's humanitarian coordination agency OCHA said water levels in the Euphrates had risen 10 meters (33 feet) since late January, in part from heavy rainfall and snow.

But it warned that damage to the dam "could lead to massive scale flooding across Raqqa and as far away as Deir Ezzor" province to the southeast.

Any further rises in the water level or damage to the Tabqa Dam "would have catastrophic humanitarian implications in all areas downstream", the UN warned.