The Islamic State group ordered residents to evacuate the Syrian city of Raqqa on Sunday following reports that a dam contested by U.S.-backed forces upstream on the Euphrates River could collapse, activists reported.
The militants said coalition airstrikes had weakened the Tabqa Dam, some 40 kilometers (25 miles) west of Raqqa, and that the water level behind the dam was rising. The extremists captured the city from Syrian rebels in 2014 and it now serves as the capital of the group's self-styled Islamic caliphate.
Civilians began fleeing midday, according to the activist-run Raqqa is Being Slaughtered Silently.
The Britain-based Syrian Observatory for Human Rights, an opposition-run monitoring group, on Sunday reported that the IS-held dam was out of service for unknown reasons.
The two groups rely on local contacts to smuggle information out of IS-held territory.
The U.S.-led coalition battling IS could not immediately be reached for comment.
The reports from Raqqa came as a leading Syrian opposition group called on the U.S.-led coalition to stop targeting residential areas in and around the city.
The Syrian National Coalition said in a statement that it was "increasingly concerned" about civilian casualties in the campaign against the extremist group. The exiled opposition coalition is taking part in U.N.-mediated talks in Geneva.
The SNC said it believed coalition forces were behind an airstrike 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 U.S. has provided substantial air and ground support to the Kurdish-led Syrian Democratic Forces, who are closing in on Raqqa as well as the Tabqa Dam.
The Britain-based Syrian Observatory for Human Rights said coalition airstrikes have killed 89 civilians in the Raqqa province in the past week, including 35 in the Badya school, in the village of Mansoura.