In another attack, rebels and opposition forces killed more than two dozen pro-regime fighters in Al Ateira, a northwestern stronghold of Syrian regime leader Bashar al Assad, according to the Syrian Observatory for Human Rights.
A suicide car bomb attack in southwestern Syria on Tuesday killed at least eight fighters, including regime and opposition forces, said the Syrian Observatory for Human Rights (SOHR).
The explosion hit Zaizun, a village in the western countryside of Daraa province, which opposition forces 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 Daesh.
Abdel Rahman said it was "likely" that the Daesh 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 regime 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 opposition for them to surrender their weapons and hand over towns to regime 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 percent of Daraa province, while opposition hold around 15 percent, according to the Observatory.
Jaish Khaled bin Walid – which is not included in the ceasefire deal – controls the remaining strip where around 30,000 people live.
Anticipating a regime attack, thousands of people have fled the Daesh-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 [Daesh] affiliate there," said Abdel Rahman.
Regime forces have not yet begun their assault but opposition forces nearby hit the Daesh-held area with artillery and mortars late Monday.
'Rebels kill 27 regime fighters in northwest Syria'
Rebels and opposition forces killed more than two dozen pro-regime fighters in a joint attack in a northwestern stronghold of Syrian regime leader Bashar al Assad's regime, the SOHR said earlier on Tuesday.
In retaliation, the regime and its ally Russia pounded opposition-held areas in the coastal province of Latakia and neighbouring Idlib on Tuesday, killing four civilians, the Britain-based war monitor said.
It was the bloodiest opposition assault on the area in three years, according to the SOHR, which relies on a network of sources inside the country.
The surprise offensive late Monday targeted a pair of villages and observation points in Latakia, near the Turkish border.
"At least 27 regime forces and allied fighters, including eight officers, were killed in fierce clashes and shelling in the village of Al Ateira," said Observatory head Rami Abdel Rahman.
"They took Al Ateira and kicked out regime forces from several observation points after killing or wounding them."
Another 40 Assad loyalists were wounded and six rebels were killed, he said.
Al Ateira lies about two km (just over a mile) south of the Turkish border.
The Observatory said the opposition forces included local rebels from Latakia.
Latakia is the heartland of Assad's regime on the Mediterranean coast, and is home to the Hmeimim airbase where Russian troops and warplanes have been based since 2015.
Forces opposed to Assad have repeatedly attacked the province since the uprising broke out in March 2011. They hold a northeastern sliver of territory bordering the neighbouring province of Idlib.
The regime and Russia bombarded that strip of land and parts of western Idlib with air strikes and artillery fire on Tuesday, killing four civilians, the monitor said.
That area falls under a de-escalation deal agreed last year between Russia, fellow regime backer Iran and opposition ally Turkey.