The UK-based Syrian Observatory for Human Rights said Russian aircraft targeted various positions overnight.
But Russia has denied carrying out operations over the city, Interfax news agency reports.
Idlib city and the province of the same name are a bastion of groups including the al-Qaeda-linked Nusra Front.
The Nusra Front is excluded from a partial cessation of hostilities brokered by the US and Russia in February to make way for a resumption of peace talks.
The Syrian Observatory said the attacks hit several residential areas. There were also strikes near a hospital and a public garden, it added, saying that seven children were among the victims.
Pictures showed buildings destroyed and rescue teams working through the night searching for casualties.
Russia is a key ally of Syrian President Bashar al-Assad and has carried out an air campaign that it says targets "terrorists".
Meanwhile, the Turkish foreign ministry said the death toll in the strikes was above 60, and that 200 people had been injured.
In a statement, it called on the international community to act against what it called the "indefensible" crimes of the Russian and the Syrian administration.
Turkey has strongly criticised the Russian operations in Syria and opposes the Assad government.
At least 23 people have been killed by air strikes on the rebel-held city of Idlib, in north-western Syria, activists say.