Syria airstrike kills top Nusra commander
A coalition of rebel groups has struck at a key border crossing between Jordan and SyriaHamid Khatib/Reuters file photo

Jordan has closed its border with Syria after rebels fighting Bashar Al Assad seized control of crossing between the two countries.

It was not clear which rebel group had taken the Nasib border post from Assad's forces, with various reports claiming that Free Syrian Army (FSA), the Al Qaeda-linked Jabat Al Nusra and another rebel faction, the Southern Front, was in control.

Other sources said that a coalition of groups calling itself the Islamic Front had captured the crossing, a major gateway for the Syrian government.

"Our fighters were able to seize the Nasib crossing after forcing pro-Assad militias to withdraw from the area," said Muhammad Horani, an FSA member, told ARA News.

"The liberation of the Nasib crossing was a result of a strategic operation by several rebel factions."

Various groups have since posted pictures on social media of their flags hanging on the border post, including that of Al Nusra, which published a picture of an Islamist fighter climbing a fence on the Syrian side of the border and hanging a black flag with white Arabic text.

Jordan said it had beefed up security on its side of the border, while the Syrian air force had started bombing the captured post. The Syrian Observatory on Human Rights claimed that Syrian planes were using barrel bombs.

The capture of Nasib comes at a critical time for Assad, whose army lost the city of Idlib to rebel groups last week and on Wednesday saw Islamic State (Isis) forces capture huge areas of the Yarmouk Palestinian refugee camp on the outskirts of Damascus.

If IS are able to hold onto the camp - where an estimated 18,000 Palestinians still live - it will bring Islamist militants the furthest inside the Assad stronghold of Damascus than ever before.