A suicide car bombing on Tuesday killed 14 newly reconciled fighters from pro-government and rebel ranks in an attack on a south Syrian village claimed by ISIS.
The Britain-based Syrian Observatory for Human Rights said the explosion hit Zaizun, a village in the western countryside of Deraa province which rebels agreed in recent days to hand over to regime control.
At least "14 regime and opposition fighters who recently reconciled were killed in a suicide car bomb attack targeting a military position in Zaizun," said Observatory head Rami Abdel Rahman, giving an updated toll.
ISIS claimed responsibility for the attack in a statement posted on the Telegram messaging app, saying the car driven by a suicide bomber had been packed with explosives.
Zaizun lies near a sliver of southern territory controlled by Jaish Khaled bin Walid, which has pledged allegiance to ISIS, which is not bound by the deal between the rebels and the government.
Mr Abdel Rahman said it was likely that this faction was responsible for the attack, adding it was the first suicide bombing targeting regime forces since they launched an offensive on Deraa.
Backed by Russia, Syrian troops began a bombing blitz of Deraa 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.
Anticipating a regime attack, thousands of people have fled the ISIS-held zone in the past two days heading toward the Israeli-occupied Golan Heights.
Around 200,000 displaced people have already sought refuge along that frontier, which remains sealed, according to the UN.
"After finishing off rebel forces in the south, the regime is expected to attack the last pocket held by the ISIS affiliate there," said Mr Abdel Rahman.
Government forces have not yet begun their assault but rebels nearby hit the ISIS-held area with artillery and mortars late on Monday.
Meanwhile Syrian state news agency SANA on Tuesday said four other villages northwest of Deraa were expected to seal deals with the government on Wednesday, citing unspecified "reports".