Rebel fighters captured a major base from the Syrian army in the south of the country on Tuesday, rebels and a monitoring group said, a setback for President Bashar al-Assad reflecting the mounting pressure on him after recent losses elsewhere.
Syrian officials could not immediately be reached for comment. The Syrian Observatory for Human Rights, a UK-based group that monitors the war, said a nearby town and a village had fallen to the rebels in addition to the base, Liwa 52.
"We announce the liberation of Liwa 52," Issam al-Rayyes, spokesman for the "Southern Front" alliance of rebel groups, told Reuters. Liwa 52, or the 52nd Brigade, is one of the biggest Syrian army bases in the area.
The southern region near the border with Jordan and Israel is one of the areas where insurgents have inflicted significant defeats on Assad in the last three months, notably by capturing the Nasib border crossing with Jordan on April 1.
Less than 100 km (60 miles) south of Damascus, the area is one of the last major footholds of rebel groups that are not dominated by hardline jihadists such as Islamic State and the Nusra Front, al Qaeda's Syrian arm, although Nusra also has a presence there.
An earlier Syrian state TV reported that the army had repelled an attempt by "a terrorist group" to infiltrate a military position in the area. It said a number of the attackers had been killed and wounded, including a rebel commander. The air force was carrying out raids in the area, it added.
The "Southern Front", an alliance of rebel groups, has been coordinating operations against Assad from a joint command center in Jordan. It includes groups that have received some support from foreign states that want to see Assad gone, including Gulf Arab governments.
Washington, which has been leading an air campaign against Islamic State since last year, says its strategy depends on encouraging the success of groups that oppose both Assad's government and the jihadists.
Saber Safar, commander of the "First Army" rebel group that said it led the attack, told Reuters by Skype that government forces had completed a "mass flight" after starting to withdraw in recent days.
The rebels had fired more than 100 missiles at the base during the attack, the opposition-affiliated Orient News TV station said.
ISLAMIC STATE ATTACKS HEZBOLLAH NEAR BORDER
Since late March, an alliance of insurgents including the Nusra Front have seized nearly all of the northwestern province of Idlib at the Turkish border. Islamic State has also seized the city of Palmyra from government control.
In both cases, the Syrian military and militias fighting alongside it have appeared to withdraw rather than fight. Analysts say the decisions to retreat point to strain in the army after more than four years of combat.
The pressure has prompted Western policymakers to suggest a window of opportunity for a political deal may be opening in Syria after a war that has killed 250,000 people and made some 8 million homeless.
But the defeats have also triggered renewed statements of support for Assad from Iran, whose backing has been crucial to his survival.
While Assad has suffered setbacks in the northwest, southwest and east, he has tightened his grip over the border zone with Lebanon thanks to a joint operation over the last month with the Iranian-backed Lebanese group, Hezbollah.
The operation underway in the Qalamoun border region has cleared insurgents from hundreds of square kilometres between Syria and Lebanon.
Hezbollah fighters came under attack from Islamic State militants for the first time in the Qalamoun campaign on Tuesday, Lebanese sources briefed on the battle said. Eleven Islamic State fighters were killed, the sources said.
