In Jassem in the southern province of Daraa, 15 civilians were killed in four air strikes, while 25 others were wounded, said the Syrian Observatory for Human Rights.
The strikes in response to a major rebel offensive that has been underway in southern Syria for months.
"As usual, the regime is striking populated areas in order to make civilian supporters of opposition fighters turn against them," Observatory director Rami Abdel Rahman said.
Rebels fighting to oust President Bashar al-Assad have suffered a spate of defeats at the hands of his forces, but they still have the upper hand in Daraa.
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Rebel fighters in the area benefit from "the fact that supply lines from Jordan are still open," he added.
The involvement of experienced fighters of Syria's Al-Qaeda affiliate, the Al-Nusra Front, has also helped the rebels to gain territory in Daraa.
Elsewhere, air strikes on Douma, the besieged rebel-held town east of Damascus, killed at least six civilians and wounded dozens more, the Observatory said.
At Khan Sheikhun in the northwestern province of Idlib, 11 people were killed, including a former army officer who had defected from the loyalist army to join opposition forces, said the Observatory.
The regime first deployed warplanes in the Syrian war in July 2012.
Now, nearly four years into the conflict, there are air strikes every day, despite repeated warnings from the international community that such tactics fail to discriminate between civilian and military targets.
On a separate front, Syrian Kurds fighting the Islamic State group made fresh advances near Kobane, seizing a new string of villages a week after the flashpoint town was recaptured from the IS jihadists.