At least 23 rebel fighters were killed on August 11 and dozens more wounded in a powerful 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 the Nassib border crossing near the headquarters of Jaish al-Islam (Army of Islam), one of the main rebel groups fighting the Syrian government in a six-year civil war.
There was no immediate claim of responsibility, but militants linked to the Islamic State group have attacked rebels in southern Syria.
The Nassib crossing used to be a major transit point into Syria from Persian Gulf countries through Saudi Arabia and Jordan, until it was seized by rebels in 2015.
The town of Nassib is southeast of the southern Syrian city of Deraa in an area covered by a U.S.-Russian brokered cease-fire that went into effect in July.