At least 32 people, including a child and 10 rebel fighters, were killed when the car blew up in front of the mosque in the rebel-held village of Yaduda in the southern Daraa province, said the Syrian Observatory for Human Rights.
Activists accused regime forces of responsibility for the attack, said the Britain-based monitor, adding that it expected the death toll to rise because many people were also wounded in the explosion.
It said the rebels had dug tunnels under and around the Carlton and they set off mines to detonate them today.
"At least five troops were killed and 18 others wounded in the attack, which also damaged parts of the hotel," said the Observatory.
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Fierce battles broke out after the attack, killing an unknown number of rebels, it added.
Insurgents fighting to topple President Bashar al-Assad's regime have used this tactic before, both in Aleppo, Syria's onetime commercial capital, and in Damascus province.
Though rebels once welcomed ISIL in the fight against Assad, horrific abuses in areas under their domination have turned much of the opposition against them.
The rebels today drove out ISIL from their positions in five Aleppo provincial towns, including Hreitan, said the Observatory.
But just before their withdrawal from Hreitan, ISIL's fighters executed 17 civilian relatives of rival Islamist rebel battalions.
In Azaz, also in Aleppo province, clashes raged between rebels and ISIL, as the jihadists "beheaded at least four" rival opposition fighters, said the group.
More than 1,36,000 people have been killed in Syria's brutal war since March 2011, and millions more have fled their homes.