Rebels backed by Syria's Al-Qaeda affiliate seized control Wednesday of the last border crossing with Jordan that had been under regime control, dealing a major blow to President Bashar al-Assad.
"Armed groups, joined today by Al-Qaeda affiliate Al-Nusra Front, took over the Nasib border point from regime soldiers" on the Syrian side of the frontier, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
A coalition of rebel groups had launched the offensive Tuesday, and Al-Nusra joined them late Wednesday.
A pro-regime security source said the "Syrian army withdrew from the Nasib border crossing."
Abdel Rahman said the takeover meant there was "no longer a regime presence on the border with Jordan."
Shortly afterwards, regime helicopters began dropping barrel bombs -- crude weapons made of oil drums or barrels stuffed with explosives -- on rebel positions near Nasib, said the Observatory director.
There was no immediate information on any casualties.
Jordan had said earlier it closed the crossing, which leads to Syria's Daraa province, to both travellers and goods.
It was a "preventive measure to safeguard the lives and security of travellers due to the fighting underway on the other side of the border," Interior Minister Hussein Majali told AFP.
Rebels have been gaining territory in the southern province of Daraa, cradle of the 2011 Arab Spring-inspired uprising against Assad that triggered Syria's civil war.
Last week they seized full control of the ancient town of Bosra al-Sham, pushing pro-regime forces out after days of heavy fighting.
In January rebels -- including Al-Nusra Front -- seized an important government army base in the province.
- IS's first Damascus foray -
In another upset for the regime, the extremist Islamic State group stormed a refugee camp in Damascus its first assault inside the capital.
Clashes between armed factions and IS jihadists raged in the Yarmuk refugee camp in southern Damascus after the jihadists overran much of the Palestinian haven in a lightening assault.
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"Fighters from IS launched an assault this morning on Yarmuk and they took over the majority of the camp," said Anwar Abdel Hadi, the Palestine Liberation Organisation's political affairs director in Damascus.
"They reached the Palestinian hospital and 15th Street, which are in the centre of the camp," he said.
The Observatory said IS took control of a "large part" of Yarmuk during fighting with Palestinian groups also opposed to Assad.
According to the Observatory, the jihadists had infiltrated the camp from the rebel-held town of Hajar al-Aswa.
But by Wednesday evening, an armed group loyal to Palestinian movement Hamas had regained some of these areas, said the Britain-based monitoring group.
It said three people had been killed in the fighting.
The UN agency for Palestinian refugees, UNRWA, said it was "extremely concerned" about the safety of civilians in Yarmuk and that some 3,500 children were in "extreme risk of death, serious injury, trauma, and displacement".
Yarmuk, located only six kilometres (3.7 miles) from downtown Damascus, was once a thriving home to 160,000 Palestinian refugees and Syrians.
But it has been devastated by fighting and a tight blockade imposed by the army nearly two years ago that created dire humanitarian conditions.
The camp, covering an area of just two square kilometres (less than a square mile), has seen its population dwindle to only about 18,000.
Last year, it witnessed scenes of desperation as thousands of people crammed into devastated streets to queue for food distributed by UNRWA.
Rebel fighters had withdrawn from Yarmuk in February 2014 under a deal that left only Palestinian anti-regime groups inside.
IS, which declared a self-styled "caliphate" last year over large parts of Syria and Iraq under its control, has fought against the Assad regime and rebel groups as it seeks to gain territory.
A Yarmuk activist said IS launched the attack after some of its members were detained following the murder there of a leader of the Palestinian Islamist movement Hamas on Monday.
In other developments Wednesday, Islamist groups attacked regime positions south of the northwestern city of Idlib, now under the control of rebels, the Observatory said.
"Islamists have taken control of one-third of Al-Mastumah, where regime forces fled after leaving Idlib," Abdel Rahman said.
The Syrian army captured a strategic position in the rebel-held town of Zabadani, northwest of Damascus on the road to Beirut.
More than 215,000 people have been killed and more than half of the country's population has been displaced since the Syrian conflict erupted in March 2011.