Air strikes killed at least 73 people in rebel-held Idlib province, including 38 in the city of Maarat al-Numan, says the Syrian Observatory for Human Rights, a Britain-based group monitoring the war.
Insurgents had previously tried to get help and supplies to fellow rebels in the city from Idlib.
The Observatory said the death toll in Maarat al-Numan included five children and six members of a single family.
The bombardment included barrel bombs, improvised ordnance made from oil drums filled with explosives and dropped from helicopters, the monitor said.
The Syrian military and Russia both deny using barrel bombs, whose use has been criticised by the United Nations.
Syria's civil war, which began in 2011, pits President Bashar al-Assad, backed by Russia, Iran and Shi'ite Muslim militias against mostly Sunni rebels including groups supported by the United States, Turkey and Gulf kingdoms.
Jihadist militants are also fighting alongside the insurgents, including Jabhat Fateh al-Sham, which has a large presence in Idlib province and was known as the Nusra Front until July when it broke its formal allegiance to al Qaeda.
Russia says its air campaign, which began in September 2015, is aimed at preventing jihadists, including both Fateh al-Sham and the Islamic State group, from gaining more territory in Syria that could be used to mount attacks overseas.
