BEIRUT, Lebanon — 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, and would be the third Euphrates dam taken by the rebels, who control two smaller facilities upriver.

But the Tabqa Dam, which the government once boasted had made Syria self-sufficient in power generation, is considered a more potent weapon in the battle for allegiances in the nearly two-year-old Syria conflict. Rebel-held areas have been systematically denied electricity by President Bashar al-Assad’s forces in their effort to turn the population against the insurgency.

Claims that the Tabqa Dam was now in rebel control came as a possible new confrontation was brewing between Turkey and Syria after a Syrian minivan exploded just inside Turkish territory at Cilvegozu, an important border crossing near the rebel-held Syrian town of Bab al-Hawa. The blast killed at least 13 people, including 3 Turkish civilians; wounded at least 28; and damaged at least 19 vehicles.