Tuesday's executions took place in the village of Rityan, north of second city Aleppo, after regime forces entered that day during an offensive aimed at cutting rebel supply lines to the Turkish border, the Syrian Observatory for Human Rights said.
The Britain-based monitor said that villagers had discovered the bodies when they returned to their homes after the regime forces withdrew a day later.
Five women and 13 rebels from the six families were among the dead.
"There was no resistance except in one house where a rebel opened fire at troops before being executed along with his family," he added.
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Activist Mamun Abu Omar said some of the bodies had been mutilated.
The brief seizure of Rityan was part of an abortive army offensive launched this week to try to encircle the rebel-held east of Aleppo and relieve two besieged Shiite villages to its north.
By yesterday all but one of the villages initially taken by government forces had been recaptured by the rebels, who include fighters of Al-Qaeda affiliate Al-Nusra Front.
While the ground offensive failed, regime warplanes kept targeting rebel areas of Aleppo city and other parts of the country.
Today, eight people - among them two women and two children - were killed when a barrel bomb hit a building in an opposition-held area of Aleppo city, once Syria's commercial capital.
Five people were also reported killed in rebel shelling of regime-held areas of the city.
The air force also killed at least seven people in rebel areas east of Damascus today, the Observatory said.