An explosion hit an opposition gathering near the Syria-Jordan border on Friday, killing at least 23 fighters and wounding dozens more, a war monitor and an activist said.
According to the OSDH, the suicide bomber blew up an explosive belt in a database used by Jaish al-Islam (army of islam) near the station border Nasib.
"De-escalation zones" were announced in May by opposition backer Turkey and regime allies Iran and Russian Federation after talks in Kazakhstan.
The attack was not claimed in the immediate future, but jihadists linked to the group islamic State (IS) have already attacked the rebels in the south of Syria.
The Nasib border crossing - known as Jaber on the Jordanian side - lies in Syria's southern Daraa province and was captured by rebel groups in April 2015. He made more than 330 000 dead and millions of displaced persons and refugees.
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Three of the "de-escalation zones" have been agreed so far: in Eastern Ghouta, the northern parts of central Homs province, and in Syria's south.
"Most of the 23 rebels killed were from Jaish al-Islam".
The fourth zone, in northwestern Idlib province, has yet not become active.
The Syrian conflict began when the Baath regime, in power since 1963 and led by President Bashar al-Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.
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