Rebels on Monday seized control of the largest dam in Syria, a vital barrier along the Euphrates River in the northern province of Raqa, the Syrian Observatory for Human Rights said.
"The rebels took control of the dam, which is still in operation. They are guarding both entrances but have forbidden the fighters from staying inside for fear the regime will bomb it," Observatory director Rami Abdel Rahman told AFP.
"This is the biggest economic loss for the regime since the start of the revolution," Abdel Rahman said of the hydraulic dam, which generates 880 megawatts of power.
He noted that while the rebels had entered the control rooms, they quickly left for fear that regime forces would retaliate by bombing the dam.
Completed in 1973 after five years of construction, it was dubbed Al-Thawra or "revolution" dam -- ironically for the 1966 military coup that brought Hafez al-Assad, the father and predecessor of the current president, to power and not for the current uprising against Bashar al-Assad.
According to the ministry of water resources website, the dam is 4.5 kilometres (2.8 miles) long, 60 metres (65 yards) high and 512 metres (560 yards) wide at its base.
It holds back Lake Assad, also named for the former ruler, a 14.1 billion cubic metre (500 billion cubic feet) man-made reservoir midway along the 2,800 kilometre (1,740 mile) Euphrates, which flows from Turkey to the north to Iraq in the east.
Rebels from the jihadist Al-Nusra Front and the Awayis al-Qurani and Ahrar al-Tabqa battalions also took over three districts in the adjacent town of Tabaqa, the Britain-based Observatory said.
The fighters met little resistance in the town, while loyalist security chiefs had fled on board military helicopters from the local airbase.
Lower ranking members of the security forces were seen fleeing from the Tabaqa airforce intelligence branch, in a video posted to YouTube by activists.
The video shows men dressed in black uniforms running across a street with their hands in the air amid barrages of gunfire and the sound of echoing blasts. Its authenticity could not immediately be verified.
"Awayis al-Qurani battalion, Al-Nusra Front and several battalions from the Free Syrian Army (FSA) taking the defectors to safety at the airforce intelligence branch," a man says from behind the camera.
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.
Tabaqa is a large and diverse town comprised of Sunnis, Alawites, Christians and Kurds. According to the Observatory, the rebels have said the residents will not be affected by the power change.
The taking of the dam is the latest in a string of key rebel victories in northern and eastern Syria in recent months.
In late November, FSA fighters seized the Tishreen dam further northwest along the Euphrates that connects Raqa to Aleppo, giving them easy passage between the provinces, which both border Turkey.
Two weeks ago, rebels captured a vital suspension bridge that straddles the Euphrates further southeast in Deir Ezzor city, cutting off the main regime supply route to Hasakeh province.
Regime troops have already been forced from the vast territory stretching from Deir Ezzor city to the Iraqi border, but the rebels have yet to take a major city in the war-ravaged country.