At least 23 people have been killed by air strikes on the rebel-held city of Idlib, in north-western Syria, activists say.
The UK-based Syrian Observatory for Human Rights said Russian aircraft targeted various positions overnight.
Turkey also blamed Russia and put the death toll at more than 60. Russia has denied any involvement.
Idlib city and the province of the same name are a stronghold of groups including 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 neighbourhoods, and areas near a hospital and a public garden.
There were seven children among the dead, it added. Dozens of other people were wounded.
However, a spokesman for Russia's Defence Ministry, Igor Konashenkov, denied any Russian involvement.
"Russian planes did not carry out any combat missions, to say nothing of any air strikes, in the province of Idlib," a statement said.
The Turkish foreign ministry, in a statement, called on the international community to act swiftly against what it called the "indefensible" crimes of the Russian and the Syrian administration.
Russia is a key ally of Syrian President Bashar al-Assad and has carried out an air campaign that it says targets "terrorists".
Turkey has strongly criticised the Russian operations in Syria and opposes the Assad government.