The explosion hit Zaizun, a village in the western countryside of Daraa province which rebels agreed in recent days to hand over to regime control.
"Eight regime and opposition fighters who recently reconciled were killed in a suicide car bomb attack targeting a military position in Zaizun," said Rami Abdel Rahman, who heads the Britain-based Observatory.
No one immediately claimed responsibility for the attack, but the village lies near a sliver of southern territory controlled by a faction linked to the Islamic State group. Abdel Rahman said it was "likely" that the IS branch, known as Jaish Khaled bin Walid, was responsible.
"This is the first suicide attack targeting regime forces since the beginning of their operations in Daraa," he told AFP.
Backed by Russia, Syrian troops began a bombing blitz of Daraa province on June 19 that killed dozens of civilians and displaced more than 320,000 people.
The onslaught came to an end on Friday, when Moscow brokered a deal with rebels for them to surrender their weapons and hand over towns to government troops.
The agreement also provides for safe passage for thousands of opposition fighters and civilians to rebel territory further north, although those transfers have not yet begun.
The regime is now in control of around 80 per cent of Daraa province, while rebels hold around 15 per cent, according to the Observatory.
Jaish Khaled bin Walid - which is not included in the ceasefire deal - controls the remaining sliver where around 30,000 people live.
Anticipating a regime attack, thousands of people have fled the IS-held zone in the last two days heading west toward the Israeli-occupied Golan Heights.
Around 200,000 displaced people have already sought refuge along that frontier, which remains sealed, according to the United Nations.
"After finishing off rebel forces in the south, the regime is expected to attack the last pocket held by the IS affiliate there," said Abdel Rahman.
Government forces have not yet begun their assault but rebels nearby hit the IS-held area with artillery and mortars late Monday.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)