Syrian army troops regained full control of an airbase in southern Sweida province on Friday, a day after rebels overran large parts of it, a monitor said.

The Syrian Observatory for Human Rights said rebels from the Southern Front alliance withdrew from the Al-Thalaa base under heavy regime fire.

"Opposition fighters withdrew from areas they seized yesterday after heavy regime aerial bombardment and the arrival of reinforcements from the (pro-regime militia) National Defence Forces and Popular Committees," said Observatory chief Rami Abdel Rahman.

He said there were casualties on both sides in the fighting, but gave no immediate figure.

Rebels from the Southern Front alliance entered the airport on Thursday, quickly seizing most of the facility.

It was the first such advance by rebels into Sweida, a province which is largely in regime hands and home to the a majority of Syria's Druze minority.

Syrian state television had denied that the airport had fallen to rebels, and the provincial governor insisted life was continuing "as normal" in the region.

The rebel advance into Al-Thalaa came after they had captured on Tuesday the 52nd Brigade base, located in neighbouring Daraa province.

Sweida province has been spared much of the fighting in Syria.

The Druze are followers of a secretive offshoot of Shiite Islam, who made up around three percent of Syria's pre-war population of 23 million people.

The community has been somewhat divided during the country's uprising, with some members fighting alongside the government and others expressing sympathy for the opposition.

Mostly, the Druze have taken up arms only in defence of their areas, and have kept out of the fighting more broadly.

Concerns for the minority's fate have grown this week with the Southern Front's advance into Sweida and reports that Al-Qaeda affiliate Al-Nusra Front killed at least 20 Druze civilians in a village in northwestern Idlib province.

The incident in the village of Qalb al-Lawzah was condemned by the Southern Front, who issued a statement Thursday pledging not to fight against the Druze.

The Qalb al-Lawzah killings also drew condemnation from UN envoy to Syria Staffan de Mistura.

"He strongly condemns such attacks on civilians, in particular communities in Syria, which are in an especially vulnerable situation amidst the ongoing violent conflict," his spokeswoman said in a statement.