At least 18 civilians were reported killed in air strikes that hit a market and an industrial zone in the Syrian rebel-held city of Idlib.
A monitoring group said Russian and Syrian government planes carried out the attacks.
The air strikes occurred despite a ceasefire brokered earlier this month by Russia and Turkey.
Idlib is the capital of the northwestern province of the same name and is the last fortress of rebel groups.
So far, neither the Syrian government nor its Russian allies have commented on the latest air strikes.
The UK-based Syrian Observatory for Human Rights (SOHR) said the bombings hit the bustling al-Hal market and the industrial zone in the city of Idlib on Wednesday.
An AFP news correspondent said several cars in the industrial zone were on fire with the drivers killed inside them.
A shop owner named Mustafa told AFP that he had just left his shop to pick up some spare parts when the strike happened.
He said he returned to find the destroyed store and its four employees trapped under the rubble.
"This is not the neighborhood I left two minutes ago," said Mustafa.
SOHR said children and rescue workers were among the victims.
The pro-opposition Step News Agency said other areas in Idlib province were also targeted.
The truce brokered by Turkey and Russia should have come into force on Sunday.
The Syrian government has said it is determined to return to Idlib province, where the strongest armed faction is a jihadist group that used to be linked to al-Qaeda.