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Russian airstrikes have destroyed Idlib National Hospital in central Syria and killed dozens, activists have said.

The bombings were part of an overnight barrage on the rebel-held province which saw at least 23 civilians killed, according to the Syrian Obervatory for Human Rights.

Aid groups have put the death toll at 50, in the latest in a series of massacres believed to have been committed by Russian and Syrian regime aircraft.

White Helmets rescue teams dug through the rubble to find survivors, and footage shared online showed the corpses of children and babies killed by the bombs.

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 airstrikes, in Idlib province," military spokesman Igor Konashenkov said in a statement.

Military analysts Bellingcat pointed out that Russian denials of atrocities have been frequent in the past, despite being later found to be culpable in the killings.

In February, a hospital by Doctors Without Borders was destroyed by four Russian rockets in an attack described by the medical group as "".

Idlib is controlled by the armed rebel coalition Jaish al-Fatah, and run by local civilian councils. 

The Nusra Front, Syria's al-Qaeda affiliate, is also known to have a presence in the province, and is not part of a ceasefire agreed in late February between Russia, the regime and rebel groups.

"The airstrikes are the most intensive on Idlib since the beginning of the truce," Syrian Observatory for Human Rights 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 overnight, as aircraft hit several residential areas and a public garden.

The monitor 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.

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

Moscow has been carrying out an aerial campaign in support of President Bashar al-Assad's forces since September last year.

It has been criticised for targeting "non-jihadist rebels", many of whom are fighting the Islamic State group in addition to the Syrian regime.

Moscow has called for other rebel groups to withdraw from areas where the Nusra Front operates, and to distance themselves from what it calls "terrorist groups".

Agencies contributed to this story.