Syria’s insurgency appeared to make further territorial gains in the south on Wednesday, antigovernment activists reported, saying that rebel fighters had seized a military base near the city of Dara’a, where the uprising against President Bashar al-Assad first began more than two years ago.

The Syrian Observatory for Human Rights, a group based in Britain that has a network of contacts inside Syria, said rebel groups had seized the air defense base of the 49th battalion near the town of Alma on the northern outskirts of Dara’a, near the Jordanian border. Last week, the rebels made advances in the nearby town of Dael, which sits on the Dara’a-Damascus highway.

Rebel fighters secured Dael after having overrun military checkpoints there, positioning them to control the highway and threaten a main conduit for the government to resupply forces in the Dara’a area. The highway is also a doorway to Jordan, an increasingly important point of entry for weapons for the insurgency.

The rebel advances in southern Syria in and around Dara’a, if not reversed by Mr. Assad’s military, would add to the patchwork of territory held by the insurgency in the north near the Turkish border and in the east near the Iraq border, as well as portions of Aleppo, the northern Syrian city that was once its flourishing commercial capital.