Russian air strikes pounded an Al-Qaeda-held city in northwestern Syria overnight, killing 23 civilians, a monitoring group said on Tuesday, but Moscow denied it was responsible.

The Syrian Observatory for Human Rights said dozens of civilians were also wounded in the raids on Idlib, a provincial capital held by Al-Qaeda affiliate Al-Nusra Front and its allies since March last year.

But the Russian defence ministry denied its aircraft had carried out any strikes on the city.

"Russian aviation did not carry out any military operations, still less air strikes, in Idlib province," military spokesman Igor Konashenkov said in a statement.

Al-Nusra is not party to a Russian- and US-brokered ceasefire that went into force on February 27 between Moscow-backed government forces and Washington-backed non-jihadist rebels.

"The air strikes are the most intensive on Idlib since the beginning of the truce," Observatory chief Rami Abdel Rahman told AFP.

"Even though Idlib is not covered by the truce, it had been relatively calm with only intermittent raids," he added.

The Observatory said five children were among those killed in the strikes, which hit several residential areas and near a hospital and a public garden.

It says it determines whether strikes were carried out by Syrian, Russian or US-led coalition aircraft based on the location of the raids, flight patterns and the types of planes and munitions involved.

Footage posted on social media by the civil defence group known as the White Helmets showed rescuers holding the limp, dust-covered body of a small boy who was among the dead.

An AFP photographer saw medical staff helping crying children at a nearby clinic.

Moscow has been carrying out an air campaign in support of its Damascus ally since September last year.

It has been criticised for targeting non-jihadist rebels as well as Al-Qaeda and its rival the Islamic State group.

Moscow has called for other rebel groups to withdraw from areas controlled by Al-Nusra and break ranks with the jihadists.

More than 280,000 people have been killed in Syria since the civil war erupted in 2011.