Syria's army, backed by allied forces and Russian air strikes, seized a key rebel town south of Aleppo on Thursday, scoring its second major advance in the province this week.

State television announced the capture of Al-Hader, a former opposition bastion near the key Aleppo-Damascus highway, just 48 hours after regime forces broke a siege by the Islamic State group of the Kweyris air base in the east of Aleppo province.

"Syrian troops and allied forces have full control of Al-Hader," a military source told AFP of the town, which is around 25 kilometres (15 miles) south of Aleppo city.

The source said troops from Lebanon's Hezbollah movement as well as Iranian forces were participating in the operation.

Both advances come after Moscow launched an air campaign in support of regime forces on September 30, prompting the army to begin offensives in several provinces.

The Syrian Observatory for Human Rights monitor confirmed regime forces had taken control of large parts of Al-Hader, though it said fighting was ongoing inside the town.

It said both Syrian and Russian warplanes were carrying out strikes in the area.

"The town is the biggest headquarters for rebel forces in southern Aleppo, and capturing it would bring the army closer to the key Aleppo-Damascus highway," Observatory chief Rami Abdel Rahman said.

Al-Hader was largely controlled by Al-Qaeda affiliate Al-Nusra Front and other allied Islamist groups, he said.

The town has been a key goal in the regime's offensive south of Aleppo city launched on October 17 with Russian air support.

Aleppo city has been divided between government control in the west and rebel control in the east since shortly after fighting began there in mid-2012.

The situation is largely reversed in the countryside surrounding the city, and a large stretch of the Aleppo-Damascus highway leading from the government-controlled portion of Aleppo is under opposition control.

- A base for new offensives -

The capture of Al-Hader comes after regime forces on Tuesday entered the Kweyris military base in the eastern Aleppo province, breaking an IS siege that had lasted for more than a year.

The Observatory said Thursday that more than 100 combatants had been killed in the battle for the base, where around 1,000 regime troops had been trapped for months.

The group said at least 60 IS jihadists and 20 regime soldiers had been killed in the fighting.

It also reported the deaths of 13 Iranian forces and eight members of Hezbollah.

The monitor said fighting was ongoing to the west and southeast of the airport on Thursday, with Russian warplanes carrying out strikes in support of government troops.

A military source told AFP Thursday that the army planned to use the airport as a forward operating base for new offensives against IS in the province.

He noted that the base is a few kilometres (miles) from the station that supplies electricity to Aleppo city, as well as to major IS strongholds in the province.

"Kweyris airport is not just an air base but an integrated military base and taking control of it will give joint Syrian and Russian forces an advance post from which to launch other operations," the source told AFP.

"The Syrian army is preparing for the start of military operations in areas where clashes have not occurred for three years," he added.

He noted in particular the base's proximity to IS-held Deir Hafer, around 10 kilometres (six miles) from Kweyris, as well as Al-Bab, a major IS stronghold to the north.

Around 20 countries and international bodies are to meet in Vienna this weekend to hammer out a roadmap for peace to end Syria's more than four-year civil war.

Neither representatives of the Syrian regime nor opposition groups are expected to attend this stage of the dialogue.