Rebels announced the capture of a strategic army base in southern Syria on Tuesday, the latest of several sweeping offensives by forces­ battling President Bashar al-Assad.

A coalition of moderate rebel factions known as the Southern Front took control of the Brigade 52 base by early afternoon, spokesman Issam al-Reis said. Brigade 52 is the largest military installation in Daraa province, which borders Jordan, and is key to the defense of northern routes leading to the Syrian capital, Damascus.

“The base has been liberated, and we are in full control,” Reis said, speaking by telephone.

The loss of the base is another blow to Assad’s forces, which have recently suffered a string of battlefield defeats. A largely Islamist rebel alliance that includes al-Qaeda’s powerful Syria affiliate, Jabhat al-Nusra, has seized most of northwestern Idlib province. The Islamic State militant group has captured territory from government forces­ in the east as well as the ancient city of Palmyra, a UNESCO World Heritage site in central Syria.

The rebel and militant advances­ have raised questions among analysts about the durability of the Assad regime, which has weathered the civil war because of military and financial support from Russia and Iran. The four-year-old conflict has killed an estimated 220,000 people and displaced millions.

The attack on Tuesday marks another success for the largely moderate rebels fighting for the Southern Front, which is backed by the West and has received weapons and money from Arab countries. The umbrella group is supported by a military operations center in Amman, the capital of U.S. ally Jordan.

In April, Southern Front fighters captured the last border crossing between Syria and Jordan that was held by the Assad government.

Emile Hokayem, a Middle East analyst at the London-based International Institute for Strategic Studies, said the fall of Brigade 52 weakens government defenses around the capital. The assault also provides a glimmer of hope for moderate insurgent groups, which have been overtaken by extremist factions on several fronts.

“The Southern Front is now showing itself as an increasingly effective buffer against Islamist rebels as well as an effective means for applying pressure on the Assad regime,” Hokayem said. The rebels control about 70 percent of Daraa and are poised to seize the provincial capital from Assad’s ­forces, he said.

Hard-line Islamist groups such as Jabhat al-Nusra and the Islamic State do not have a strong presence in Daraa, partly because of tight border control by Jordanian authorities. This has curtailed the flow of extremist fighters into the area.

Hokayem said this has also helped the Southern Front expand its control in Daraa. He noted that the group could be a key player in a broader rebel advance on Damascus, which is less than 100 miles from Daraa.

“You cannot mount a big attack on Damascus unless you have the cooperation of the Southern Front,” he said.

Reis, the Southern Front spokesman, described the assault on Brigade 52 as a surprise raid that began early Tuesday. More than 2,000 rebel fighters participated in the attack, which involved storming the facility from every direction, he said. Reis estimated that Southern Front forces­ killed seven army officers and 76 pro-government fighters. He said he did not have figures for rebel casualties.

Reis said the insurgents seized “substantial war spoils,” including tanks and heavy weaponry. He indicated that Southern Front fighters would attempt an advance toward Damascus.

“We have most of Daraa liberated, our lines of defense behind us are solid, and now we can start the operation toward Damascus and the highway leading to it,” he said.

Syrian government officials could not be reached for comment on Tuesday's assault.

Sam Alrefaie contributed to this report.

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