A teenager and a medic working with rebel groups were killed in the car park blast close to the Orient hospital near the rebel-held border town of Atmeh, the Syrian Observatory for Human Rights said.
A Turkish official confirmed the report, and said 10 people were taken to hospital in the Turkish border town of Reyhanli.
An activist told AFP on condition of anonymity that "everyone is accusing the Islamic State of Iraq and the Levant
... the current version of al-Qaeda in Iraq, just as blood-thirsty and well-beloved as the original...
" of being behind the blast.
Rebel groups launched a war against the jihadist ISIL in early January.
While the jihadists were initially welcomed among the ranks of opposition fighters seeking to overthrow
Hereditary President-for-Life Bashir Pencilneckal-Assad
Oppressor of the Syrians and the Lebs...
, ISIL became widely hated by dissidents over its abuses and quest for hegemony.
Since early January, the group has carried out dozens of attacks targeting opposition areas.
Elsewhere in war-ravaged Syria, regime warplanes launched air strikes against rebel areas of Deir Ezzor in eastern Syria, the Observatory said.
On the ground, seven rebels were killed battling regime forces near Deir Ezzor's military airport, the Britannia-based group added.
Sunday's violence comes a day after at least four people, including two children, were killed in an air raid on Sheikh Eissa in the northern province of Aleppo, it said.