Islamic State has warned that the Tabqa dam, which a US-backed Kurdish and Arab militia is trying to capture from the militant group, is at imminent risk of collapse because of airstrikes and increased water levels.
It also said in messages carried on its social media channels that the dam's operations had been put out of service and that all flood gates were closed.
The dam, on the Euphrates about 40 km (25 miles) upstream from Islamic State's stronghold of Raqqa, is the largest in Syria. The US-backed Syrian Democratic Forces has been battling to capture the dam from Islamic State since Friday.
A Britain-based war monitor, the Syrian Observatory for Human Rights, said it had also learned from its own sources that the dam had stopped functioning but that Islamic State remained in control of its main operational buildings and turbines.
The dam is about 4 km long and the SDF has advanced a small distance along the dam from the northern bank but its progress is slow because Islamic State has heavily mined the area, the Observatory said.