"Twenty civilians, including 16 children, were killed in an air strike in Kafr Batikh in Idlib province," the Syrian Observatory for Human Rights said.
Rami Abdel Rahman, the head of the Britain-based monitor, said the strike hit near a checkpoint held by Hayat Tahrir al-Sham, a jihadist group that consists largely of ex-members of a former Al-Qaeda affiliate.
It was not clear whether the air strike was carried out by the Syrian regime or its Russian ally.
Abdel Rahman said the strike hit children coming out of a nearby school.
"The oldest among them is 11," he said.
In Kafr Batikh, the children's devastated families wrapped the tiny bodies of their children in thick wool blankets.
They adorned their makeshift shrouds with olive branches and other greenery from trees in the village orchard before lowering them into graves deep in the red earth.
A teacher said he was in the school with pupils when a Russian war plane launched strikes.
Abdulrahman al-Omar said the students were hurrying home when more bombing raids hit.
"Some elderly people brought them into a cave to take shelter," Omar told AFP.
As rescue workers rushed to help the victims and clear away the bodies of the first strike, the warplane hit yet again, he said.
It was the second time in a week that schoolchildren were killed in bombing raids in Syria.
On Friday, an air strike on a school in a rebel-held town outside Damascus killed 15 children and two women who were taking shelter in its basement.
More than 350,000 people have been killed since Syria's civil war started in 2011 with the brutal repression of anti-government protest.
