BEIRUT — Fresh from declaring that they had seized an important military airport and an air defense base just outside Damascus, Syrian rebels on Monday said they overran a hydroelectric dam in the north of the country.

It is the latest in a monthlong string of tactical successes that demonstrate the rebels’ ability to erode the government’s dominance in the face of withering aerial attacks. The dam supplies electricity to several parts of Syria, the activists said, and lies on an axis between the northern provinces of Raqa and Aleppo, apparently broadening the rebels’ potential supply lines in northern Syria.