The bombardment struck the town of Jayrud, 60 kilometres (35 miles) from Damascus, where the army says Islamist rebels killed a pilot they captured after he was forced to eject on Friday.
In its Friday statement about the incident, the military said the attack "will not go unpunished."
Observatory head Rami Abdel Rahman said it was the first bombardment of the town in at least two years.
"Prominent figures in Jayrud have had a local truce with the regime for at least two years, and neither fired on each other," Abdel Rahman told AFP.
He could not specify how many of the 20 killed were civilians but said at least two were medical staff.
Rebel groups in Jayrud include the Saudi-backed Jaish al-Islam (Army of Islam), the hardline Ahrar al-Sham, and Al-Qaeda affiliate Al-Nusra Front.
Jaish al-Islam spokesman Islam Alloush said on Friday that Al-Nusra Front had executed the air force pilot.
Several government aircraft have been shot down by rebels or crashed because of technical faults since the civil war erupted five years ago.
The conflict has killed more than 280,000 people and displaced millions.
The bombardment struck the town of Jayrud, 60 kilometres from Damascus.
Two medics were among at least 20 people killed on Saturday in Syrian government shelling of a rebel-held town northeast of the capital, the Syrian Observatory for Human Rights said.
