The Tabqa Dam was completed in the 1970s with the help of thousands of Syrian and Soviet workers. It houses a hydroelectric power station.
The dam sits at the end Assad Lake, a man-made reservoir on the Euphrates River named after the father and son who have ruled Syria for the past four decades.
Opposition groups distributed videos of Syrian rebels seizing caches of weapons and ammunition from positions once held by Syrian security forces.
They also showed video of what appeared to be the dam itself, towering over the surrounding countryside.