Syrian rebels said they attacked an important military base in the south and checkpoints in the city of Deraa on Thursday, trying to regain ground lost to President Bashar al-Assad's forces near the Jordanian border.
The rebels were thrown onto the defensive last week when Assad's troops retook the town of Khirbet Ghazaleh on the main north-south highway between Damascus and Jordan.
They said hundreds of fighters with rocket launchers and anti-aircraft guns were brought in on Thursday to lay siege to the fortress-like headquarters of the Syrian army's 52nd Mechanised Brigade, one of the largest bases in Deraa province.
The base, almost 80 km (50 miles) south of Damascus, lies at the heart of a heavily fortified zone which has traditionally formed a southern line of defense protecting the capital.
Since the two-year revolt against Assad erupted the garrison has been reinforced by artillery and tanks moved from smaller outposts overrun by rebels. Rebels and regional military experts say it has been used to pound rebel-held villages and towns along the frontier with Jordan in recent months.
"This battle has been a response to the overrunning of Khirbet Ghazaleh by regime forces, to open a new front and prevent any further advances," Faeq Aboud, a spokesman for the Moatasem B'Allah Battalion, part of the Faluja Hauran brigade, a rebel group participating in the assault, told Reuters.
The Hauran Plain, which extends to the Israeli-occupied Golan Heights, is the birthplace of the revolt against four decades of family rule by Assad and his late father which erupted in the city of Deraa in March 2011.
The fortified base of the 52nd Mechanised Brigade, on the outskirts of the town of Herak, has defied countless attempts to overrun it but rebels said Thursday's attack was the biggest so far.
"When we widen the fight against them we weaken the ability of the regime forces," said Abu Thaer, a rebel fighter from the Liwa al-Tawheed brigade. He said two major checkpoints in the old quarter of the city of Deraa were also attacked on Thursday after a month-long lull.
The rebels hold large areas in the north and east of the country but Assad's forces have waged a series of attacks in eastern Damascus, Homs and in Deraa, trying to consolidate control in the heartlands of Assad's power.
Syria's civil war has killed more than 90,000 people, according to the Britain-based Syrian Observatory for Human Rights, and the battle between mainly Sunni Muslim rebels and forces loyal to Assad from the Alawite minority linked to Shi'ite Islam has becoming increasingly sectarian.
The Observatory sharply raised its death toll on Thursday from an attack two weeks ago in a Sunni Muslim district of the coastal town of Banias, saying 145 people were killed by pro-Assad Alawite forces in a "sectarian massacre".
Thirty-four of those killed were children, it said.
The group, which monitors Syrian violence through a network of activists inside Syria, had previously put the toll at 62 but said that dozens of bodies were found over the following days in burnt-out houses and under the ruins of buildings.
Thursday's rebel counter-attack in the south coincided with a rebel campaign east of Damascus to retake the town of Otaiba, which is crucial to weapons supplies reaching the capital, and the area around it. Rebels said on Thursday they captured the village of Qaysa, about 3 km (2 miles) west of Otaiba.
The fall of Khirbet Ghazaleh last week checked the swift recent advances by rebels who had opened up a southern front against Assad's increasingly stretched ground forces.
The rebels, who include jihadi fighters from the al Qaeda-linked Nusra Front and others operating under the umbrella of the Free Syrian Army, cut off the highway to Jordan over two months ago. But keeping the road off-limits to Assad's forces depended on retaining Khirbet Ghazaleh.
If the army pushed further south and took the towns of Ghereyah and Ghereyah East along the main highway it could reopen the major transit route and regain part of the border strip with Jordan now in rebel hands, they said.
"The situation is now under control and rebels positioned there are aware of how dangerous the situation is. If they retake these towns, it would be over completely and control would go back to the regime in areas they had withdrawn from or been defeated," Aboud said.