BEIRUT — Syrian insurgents and opposition activists said Monday that rebel forces had taken control of Syria’s largest hydroelectric dam, an assertion that, if confirmed, would give them significant control over a vital reservoir and what remains of the sporadic power supplies in their war-ravaged country.

The Tabqa Dam, built more than 40 years ago with Russian help on the Euphrates River in northeast Syria’s Raqqa province, provides electricity to areas that are both in rebel and loyalist hands, including the contested city of Aleppo. It would be the third Euphrates dam taken by the rebels, who control two smaller facilities upriver.