Russia has denied its planes carried out air strikes overnight against the Syrian rebel-held city of Idlib, which the Syrian Civil Defence says has killed at least 50 people.

"Russian planes did not carry out any combat missions, to say nothing of any air strikes, in the province of Idlib," Igor Konashenkov, a Russian Defence Ministry spokesman, said in a statement on Tuesday.

The UK-based Syrian Observatory for Human Rights said earlier on Tuesday the death toll was 23, including seven children.

Turkey also accused Russia of killing at least 60 people in its air strikes on Idlib.

Ten overnight strikes on Monday in Idlib hit areas around the National Hospital and other parts of the city centre, according to Rami Abdul-Rahman, head of the Observatory.

"The air strikes are the most intensive on Idlib since the beginning of the truce," Rahman told the AFP news agency. "Even though Idlib is not covered by the truce, it had been relatively calm with only intermittent raids."

The strikes on Monday evening on the north-western city, which is held by a coalition of rebel groups, including al-Nusra Front, come five days after Moscow said it was suspending strikes on Syrian "gunmen".

The Russian Defence Ministry had said the pause was to allow groups who signed up for a US and Russian-backed cessation of hostilities a chance to cut their links with the al-Nusra, a powerful Syrian group with ties to al-Qaeda.

"We have been hearing from different sources on the ground that more than eight areas have been targeted by Russian air strikes," Al Jazeera's Hashem Ahelbarra reported from Gaziantep in Turkey. "One area is close to a hospital. Because of that explosion, half of the hospital was destroyed."

Ahelbarra said there were concerns that the death toll could climb as a rescue operation was under way.

Searching the rubble

"Idlib is a major opposition stronghold run by a coalition of military factions, including conservative parties such as Ahrar al-Sham. There is also a strong presence by al-Nusra Front," he said.

"The opposition says Russians are taking advantage of a ceasefire in place since February, and are trying to undermine defence lines of the opposition."

Idlib is considered the de facto capital for rebels.

Rebels have accused Russia of repeatedly targeting groups not linked to ISIL or al-Qaeda, while the US has said that battlefield alliances between al-Nusra and other rebels have complicated implementation of the truce, especially around the key northern city of Aleppo.

A volunteer with the Syrian Civil Defence, also known as the White Helmets rescue group, said the strikes appeared to have been carried out by Russian aircraft.

Rescue workers were searching for survivors under the rubble of the bombed buildings, the volunteer, who asked to be known only as Majed, told the dpa agency.

The Syrian conflict, which began with peaceful protests in March 2011, has spiralled into a multi-sided civil war.

The death toll has risen to more than 250,000 people while half the country's population have been forced from their homes, according to UN estimates.