Over 40 people were killed and scores wounded on Sunday in a suspected Russian air strike on a crowded marketplace in Idlib province, activists said.
The early morning attack occurred in the town of Ariha, 15km south of Idlib city. Local news channel Ariha Youm reported cluster bombs were used in the raid by a Russian fighter jet.
The pro-opposition Orient TV also reported an initial death toll of 40.
However, Rami Abdulrahman, director of the Syrian Observatory for Human Rights, put the death toll much higher, saying at least 60 people were killed and wounded in the attack.
Officials at the Russian defence ministry could not immediately be reached for comment.
The Russian air force has conducted air strikes in support of President Bashar al-Assad since September 30.
Ariha is located in Idlib province, which is controlled by rebel groups including the al-Qaeda-linked Nusra Front.
The province is not a stronghold of the Islamic State of Iraq and the Levant (ISIL) group that controls wide areas of eastern Syria.
The Syrian military withdrew from Ariha in May as alliance of rebel groups – including Nusra Front – advanced in Idlib in an offensive that resulted in the entire province falling into rebel hands.