Among the dead in the blast in the rebel-controlled town of Namar were a woman and a child, the Britain-based monitoring group said.
In northern Syria, the Observatory 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.
North of Aleppo city, the Observatory said jihadists from the Islamic State (IS) group extended their advances, seizing Baghaydin village near the border with Turkey.
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 — Aazaz and Marea.
IS counters backlash
Marea is a stronghold of the Islamic Front, a coalition of Islamist groups that is among those fighting against IS.
Aazaz sits next to a border crossing with Turkey, which would be a valuable asset as IS seeks to expand its self-declared “caliphate” in the territory it holds in Syria and neighbouring Iraq.
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 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 Friday 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.
In an opinion piece published on CNN’s website, Charles Lister of the Brookings Doha Centre said Syria’s rebels were facing an “existential threat”.
Lister said IS was likely to move into Aazaz, 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 farther north, the remaining rebels inside Aleppo are increasingly beleaguered.
Fighting in the province has left civilians there “living in appalling conditions”, the International Committee of the Red Cross said on Friday.
The ICRC said it was working with Syria’s Red Crescent to deliver medical aid to all parts of the province, in both rebel- and regime-held territory.
“Stepping up aid distributions in the governorate has been the ICRC’s priority ever since we established a permanent presence in the city over a year ago,” it said.
More than 170,000 people have been killed in Syria since the beginning of its conflict in March 2011, according to the Observatory.