An explosion hit a rebel gathering near the Syria-Jordan border on Friday, killing at least 23 fighters and wounding dozens more, a war monitor and an activist said.
The Syrian Observatory for Human Rights said a violent explosion hit an army camp of the Syrian rebel group Jaish al Islam near the border crossing town of Nassib.
The Britain-based war monitor said the explosion was caused by a suicide bomber and the death toll was expected to rise.
"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.
A video passed to Reuters by an activist in the area claimed to show at least a dozen seriously injured people lying on a carpeted floor. Reuters could not independently verify the content of the video.
The Nassib crossing used to be a major transit point into Syria for people and goods coming to and from the Gulf countries through Saudi Arabia and Jordan, until it was seized by the opposition during Syria's civil war.
The town of Nassib is southeast of the southern Syrian city of Deraa in an area covered by a US-Russian brokered ceasefire that went into effect in July.
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.