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The Islamic State group on Tuesday claimed it had killed 35 Syrian regime and Russian troops in a car bombing in the southern province of Daraa, which rebels agreed in recent days to hand over to regime control after Assad's forces launched a large-scale offensive to oust opposition forces.

A final toll has not yet been announced by Syrian regime or Russian authorities.

IS claimed responsibility for the attack in a statement posted on the Telegram messaging app, saying a truck driven by a suicide bomber had been packed with explosives, targeting the village of Zaizoun in eastern Daraa province.

The statement added that the explosion wounded other troops, including Russian forces, and destroyed two tanks.

However, the Britain-based Syrian Observatory for Human Rights said the explosion killed 14 regime and opposition fighters who had recently "reconciled" and signed a ceasefire deal.

Zaizun lies near a sliver of southern territory controlled by Jaish Khaled bin Walid, a extremist faction that has pledged allegiance to IS, which is not bound by a ceasefire deal agreed between the rebels and the regime.

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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 Daraa.

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 percent of Daraa province, while rebels hold around 15 percent, according to the Observatory.

Anticipating a regime attack, thousands of people have fled the IS-held zone in the past two days heading toward the Israeli-occupied Golan Heights.

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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.

Regime forces have not yet begun their assault but rebels nearby hit the IS-held area with artillery and mortars late Monday.

Meanwhile Syrian state news agency SANA on Tuesday said four other villages northwest of Daraa were expected to seal deals with the government on Wednesday, citing unspecified "reports".

Agencies contributed to this report.