The reported fall of the Mayadeen base, along with its stockpiles of artillery, caps a series of advances in Deir el-Zour including last week's seizure of a military airport.
The province borders on western Iraq. Syria's rebels enjoy strong support with the Sunni tribes of Iraq's west, and many Iraqis with combat experience from their own war are believed to have crossed to fight in their neighbor.
Rebel fighters also say that weapons seized when bases fall have been essential to their transformation from ragtag brigades into forces capable of challenging Assad's professional army.
Activist groups and a local fighter told The Associated Press the Mayadeen base was taken in the morning hours, after a three-week siege. The fighter spoke on condition of anonymity because of security concerns.
Violence also was reported in opposition strongholds around the capital Damascus and in the northern city of Aleppo, where government aircraft damaged one of the rebels' key field hospitals.
Rebels who have battled government forces for months to control Aleppo, Syria's economic hub, scored a major victory several days ago when they overran the nearby base of the regime's 46th Regiment. The unit was a pillar of the government's Aleppo garrison and its fall cuts a major supply line.
However, the regime has used its air power to dent rebel gains. Government aircraft late Wednesday flattened a building next to Dar al-Shifa hospital, killing 15 people and badly damaging one of the last remaining sources of medical help for civilians in the city, activists said.
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