"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, head of the Syrian Observatory for Human Rights.
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Abdel Rahman said regime troops had withdrawn from Nasib, leaving no Syrian government presence along the entire border with Jordan.
There was no immediate information on any casualties.
The Jordanian government had said earlier that it had closed the crossing on its side of the frontier to both travellers and goods.