At least 23 rebel fighters were killed Friday and dozens more wounded in a suicide blast in southern Syria near the border with Jordan, a monitoring group said.
The Syrian Observatory for Human Rights said the bomber detonated an explosive belt at a base used by Jaish al-Islam (Army of Islam) near the Nasib border crossing.
"Most of the 23 rebels killed were from Jaish al-Islam. Dozens were wounded, including 20 in critical condition," said Observatory head Rami Abdel Rahman.
There was no immediate claim of responsibility, but jihadists linked to the Islamic State group have attacked rebels in southern 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.
Syria's uprising erupted in Daraa province in March 2011 with widespread protests calling for the ouster of President Bashar al-Assad.
It has since turned into an all-out war that has drawn in international powers and killed more than 330,000 people.
Under a plan hammered out in May between Russia, Turkey and Iran at peace talks in Kazakhstan, four "de-escalation zones" were to be established across swathes of Syria to ease fighting between regime and rebel forces.
Parts of southern Syria make up one zone. Another lies in the rebel bastion of Eastern Ghouta near Damascus, while a third is in the central province of Homs.
The fourth zone, in northwestern Idlib province, has yet to be implemented.
