The Britain-based monitoring group said the death toll for the bombing in the rebel-controlled town of Namar was likely to rise because of the high number of serious injuries.
Among the dead were a woman and a child, the group said.
In northern Syria, the group reported 10 people killed when regime helicopters dropped explosive-packed "barrel bombs" on Aleppo city.
The attacks targeted the Bab al-Nairab district of the Old City and the southern Salahin neighbourhood.
On Wednesday, IS fighters captured eight villages in the area between Aleppo and the border from rival rebel groups.
Those advances came after clashes that killed at least 40 fighters from rebel groups, as well as 12 IS fighters, according to the Observatory.
The successes open the way for IS to move towards two strategic prizes -- Azaz and Marea.
Marea is a stronghold of the Islamic Front, a coalition of Islamist groups that is among those fighting against IS.
IS emerged from Al-Qaeda's one-time branch in Iraq, and initially fought alongside Syria's opposition, including more moderate rebels and Al-Nusra fighters.
But its abuses and harsh brand of religion prompted a backlash from rebel groups that pushed it out of many of opposition-held areas earlier this year.
Since then, it has staged a comeback, bolstered by weaponry obtained in advances across the border in Iraq.
One analyst warned today that the fate of Syria's "Western-backed opposition in northern Syria hangs quite literally on a knife-edge" as IS advances in northern Aleppo and regime forces advance in Aleppo city.
Lister said IS was likely to move into Azaz, cutting rebel supply lines to Syria, and into Marea, from which it could launch attacks on rebel-held parts of Aleppo city.
Aleppo has been divided between rebel control in the east and regime control in the west since shortly after fighting began there in mid-2012.
In the face of regime advances around the eastern outskirts of the city, coupled with IS advances further north, the remaining rebels inside Aleppo are increasingly beleaguered.
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