The artillery base is near the town of Mayadeen in Deir al-Zour province, the British newspaper The Daily Telegraph reported. Gaining control of the area would allow the rebels to bring in weapons and fighters from Iraq.
"The military base was liberated early this morning. It was under siege for 15 days," an activist using the name Mohammed Saleh said Thursday. "This is the last military base in the countryside. Now the territory is liberated all the way to the borders with Iraq."
The province is also the location of much of Syria's oil, and the rebels now control two of the three biggest oil fields, the Telegraph said.
Saleh said the government of President Bashar Assad has withdrawn troops from the area to shore up the defenses of Damascus. But government forces remain in Deir al-Zour city, the sixth-largest in the country, and fierce fighting continued there, rebels said.
"There is shelling night and day. The bridges that cross the Euphrates River and connect the city together have been targeted by air strikes and destroyed," a young man using the name Abu Wissam told the Telegraph.
The Syrian Network for Human Rights reported 61 deaths across Syria including seven children. Twenty-two people were reported killed in Damascus and the surrounding area, 12 in Aleppo, five in Hama, four each in Daraa, Lattakia, Deir al-Zor and Idlib, two each in Homs and Raqqa and one each in Tartus and Kenitra.