The Syrian government has responded to either report.
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.
The raids in the area were the first since a truce was announced one week ago. He made more than 330 000 dead and millions of displaced persons and refugees.
Air strikes have also pummelled rebel-held parts of the Jobar district of Damascus and the adjacent district of Ain Tarma.
The fourth zone, in northwestern Idlib province, has yet to be implemented.
A bomber detonated an explosive belt at a base used by Jaish al-Islam near the Nasib border crossing on Friday, the war monitor group said.
The post-borders of Nasib, called Jaber jordanian side, is located in the province of dara'a, conquered by rebel groups in April 2015.
There was no immediate claim of responsibility, but jihadists linked to the Islamic State group have attacked rebels in southern Syria.
Sweida province was not included in a US-Russian brokered ceasefire that took effect in nearby areas of the southwest in July.
"Most of the eastern Sweida countryside is now in the hands of the regime", he added.
"Most of the 23 rebels killed were from Jaish al-Islam".
Syria's uprising erupted in Daraa province in March 2011 with widespread protests calling for the ouster of President Bashar al-Assad.