The rebels said they came under attack at dawn in a sparsely populated desert area that lies east of the pro-government controlled city of Sweida, mainly inhabited by the Druze minority.
The air and ground offensive, backed by Russian air power, was waged on eight villages from Tal Asfar to Tlul al Shuhaib that had been seized at the end of March by Free Syrian Army (FSA) rebels from Islamic State after the hardline militants had retreated to regroup further north.
"This is the biggest attack by the regime and on the villages of eastern Sweida. They have used all types of weapons from aerial bombing to artillery and an unprecedented ground offensive," said Mohammad Adnan, spokesman of Jaish Ahrar al Ashaer, a rebel group composed of tribal fighters operating in the border area with Jordan.
"In the years when Daesh controlled these area, the army never clashed with them," Adnan added.
Adnan later said the army was able to seize at least seven villages. An army statement said they had recaptured several towns, villages and strategic hilltops in the eastern Sweida countryside.
Bedouin tribes
The rugged area east of Sweida is mainly inhabited by Bedouin tribes who have long defied government authority. Many of them make up the main rebel groups based in the area.
The area is not part of the US-Russian brokered ceasefire for southwest Syria that came into effect on Sunday in the first peace-making effort of the war by the US government under President Donald Trump.
In the southwest, where the ceasefire was generally holding, rebels said the army had stopped aerial bombardments but intermittent shelling continued on rebel-held areas in Deraa city and in the Syrian Golan Heights.
"We are monitoring violations by the regime in several areas using mortars and heavy artillery," said Major Issam al Rayes, spokesman for the Southern Front coalition of Western-backed rebel groups.
The Syrian army backed by Iranian-backed militias have since June stepped up their bombardment of rebel-held areas in the border city of Deraa in a bid to reach the Jordanian border.
It has also been sending reinforcements further towards the southeastern desert to prevent areas left by Islamic State from falling into the hands of the FSA.
"The Iranian Shia militias are continuing to pour reinforcements into the Badia to expand their area of control. We are doing what we can to repel them," said Saed Saif, an official from the FSA's Ahmed Abdo Martyrs Brigade.
The advances in areas that had been in Islamic State hands have brought them within reach of the Iraqi frontier for the first time in years. They have also effectively encircled FSA-controlled desert territory stretching to the Jordanian and Iraqi borders and close to the Tanf garrison in Syria where US forces are based.
