Al-Nusra Front, the Al-Qaeda franchise in Syria, has seized a moderate rebel commander in the southern province of Daraa, a monitoring group said on Sunday.
"Al-Nusra Front last night arrested the head of the Military Council of Daraa, Captain Ahmad Naameh, along with five other commanders of opposition factions," said the Syrian Observatory for Human Rights.
Activists also reported his capture, which took place amid significant rebel advances in the province, where Al-Nusra Front has only a limited presence.
Naameh had travelled from Jordan to Daraa last week to help unite rebels fighting to topple President Bashar al-Assad's regime, excluding those of the jihadist Al-Nusra.
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In a video recorded this week, distributed by the Observatory, Naameh said: "Who is going to rule Syria? Not the extremists... who behead people... No, it will be the Free Syrian Army, which is well organised, and which believes in democracy, democratic rule, and the civil state."
The Observatory's Rami Abdel Rahman told AFP he was likely to have been detained because of the statement.
But some activists in Daraa have accused Naameh of being power-hungry.
Some two months ago, rebels fighting to topple Assad established the so-called Southern Front, which includes some 30,000 fighters from more than 55 mainstream opposition groups operating from the Jordanian border to the outskirts of Damascus and the Golan Heights.
The new alliance is in part aimed at alleviating Western concerns that providing greater aid to the fractious rebels would bolster Al-Qaeda groups and see heavy weapons fall into the hands of extremists.