Syrian rebels took control of army checkpoints guarding the entrances to the critical Euphrates River dam in the northeastern province of Raqa on Monday, a watchdog said.
"There is little resistance in the area and it looks like the battalions will push through to the control rooms of the dam in the coming hours," the Syrian Observatory for Human Rights reported.
Rebels from the jihadist Al-Nusra Front and the Awayis al-Qurani and Ahrar al-Tabqa battalions took control of three districts in the town of Tabqa on the Euphrates and captured a number of security force members, it said.
"If the rebels are able to control the Euphrates dam, it will be a painful blow to the regime," the watchdog said.
In a video posted by activists to YouTube, small groups of men dressed in black uniforms can be seen running across a main street of Taqba with their hands in the air amid barrages of gunfire and the sound of echoing blasts.
"Awayis al-Qurani battalion, Al-Nusra Front and several battalions from the Free Syrian Army taking the defectors to safety at the airforce intelligence branch," a man says from behind the camera.
The video, apparently filmed in Tabqa, could not immediately be verified.
On Sunday, the rebels overran an army base in the town, securing a major cache of artillery and ammunition and taking control of a vital checkpoint.
The rebels control large swathes of territory in northern and eastern Syria but have made little headway in major Syrian cities, where military stalemates have persisted for months.
Two weeks ago, the insurgents gained control of the main suspension bridge that straddles the Euphrates river in Deir Ezzor city, severing a key army route to the northeast Kurdish areas of the country.
The United Nations puts the number of people killed in Syria's 22-month conflict so far at more than 60,000.