At least 23 people were killed in Russian air strikes overnight in the rebel-held Syrian city of Idlib, the heaviest bombardment there since a cessation of hostilities was agreed in February, the Syrian Observatory for Human Rights said.

The Observatory said the air strikes targeted a number of positions in the city, one of them next to a hospital. Seven children were among the dead, Observatory Director Rami Abdulrahman said. The toll was likely to rise with scores of injured, mostly civilians, in a critical condition.

Residents of Idlib said hundreds of families had begun leaving the city for fear of further air raids. Two rebel sources said Russian jets also intensified strikes on towns in rural Idlib including Binish, Maarat Misreen and Saraqeb.

However, a Russian Defence Ministry spokesman on Tuesday denied it had conducted air strikes overnight against Idlib, calling the Observatory's allegations "a horror story" that should be regarded with scepticism.

Russia deployed warplanes to Syria last year to support President Bashar al-Assad against rebels seeking to end his rule.

The Turkish Foreign Ministry said the air strikes had killed more than 60 civilians and complained in a statement about what it said were the "indefensible" crimes of the Russian and Syrian governments. It gave no indication how it arrived at the higher toll.

Later the monitor reported that unidentified jets bombed a major camp of the powerful Islamist Ahrar al Sham insurgent group in Syria's northwestern province of Idlib, leaving a large number of dead and wounded.

The Observatory, which tracks violence across the country, said top trainers from among Ahrar al Sham's leaders were normally present at the camp, located in the Sheikh Bahar area of rural Idlib. A senior member of the group, who requested anonymity because he was not authorised to comment, said the report was unfounded but gave no details.

Rescue workers searched for casualties through the night, finding some survivors including a child under the rubble of buildings, the Civil Defence said on its Facebook page.

Idlib city and the province by the same name is a stronghold of rebel groups including the al Qaeda-linked Nusra Front.

The U.N.-backed Feb. 27 cessation of hostilities agreement was intended to allow an opportunity for peace talks and delivery of humanitarian relief across Syria.

However, peace talks in Geneva aimed at ending the war have so far failed and the cessation of hostilities agreement has all but collapsed with intensified fighting among warring parties.

In Aleppo city, the Syrian army stepped up its bombing and shelling of the Castello highway, the only route for civilians and rebels in and out of the city.

"The artillery shelling has affected the traffic on the road but they have not taken it," said Captain Abdul Salam Abdul Razaq, a military spokesman for Nour al Din al Zinki, a main rebel group operating in northern Syria.

The Syrian army and allied fighters failed in three major offensives in the past month to advance toward the highway from the village of Handarat, several kilometres to the northeast, where rebels have been holding their ground, he added.

The rebel held towns of Anadan, Hreitan and Maarat al-Artieq just north of Aleppo city that are located along the rebels' supply route, also witnessed intense raids by Russian and Syrian fighter jets, Abdul Razaq said.

In southern suburbs of Damascus, the Syrian army made new advances in an area that they had seized almost two weeks ago, the Syrian Observatory and rebel sources said.

The monitor said army had taken large parts of the town of Mahamadieh and Beit Nayem, further consolidating gains made with the help of Lebanese Hezbollah fighters.

The areas seized are mainly lush agricultural land that provide food for tens of thousands of residents.