Syria's state news agency SANA said government troops had seized the Brigade 82 base and were clashing with rebels inside the town of Sheikh Miskeen in Daraa province.
The Syrian Observatory for Human Rights monitor also reported the advances, saying army forces backed by fighters from Lebanon's Hezbollah movement were engaged in fierce clashes backed by air strikes on Tuesday.
The Britain-based group said regime forces took the northern part of Sheikh Miskeen on Monday and seized the base on Tuesday.
Rebel groups, including Al-Qaeda affiliate Al-Nusra Front, had been in control of Sheikh Miskeen and Brigade 82 since last year.
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Abdel Rahman said the town was strategically important because of its location on a crossroads leading north to Damascus and east to the city of Sweida, both of which are under government control.
It also lies some 12 kilometres (seven miles) from the rebel stronghold of Nawa, another key target for regime forces.
Most of Daraa province is controlled by opposition forces, though the government holds parts of the provincial capital and a few villages in the northwest of the province.
Daraa is often dubbed the "cradle" of the uprising against President Bashar al-Assad that began with protests in March 2011.
Since then, more than 250,000 people have been killed in a complex, multi-front war that has displaced more than half the country's population.