Air strikes believed to have been carried out by Russian warplanes killed scores of people in the centre of the rebel held city of Idlib in northwest Syria on Sunday, rescue workers and residents said.

They said at least six strikes had hit a busy market place in the heart of the city, several government buildings and residential areas. Rescue workers said they had confirmed 43 dead but that at least 30 more bodies had been retrieved that had still to be identified. Over 150 people were wounded with some of the serious cases sent to hospitals in Turkey.

"There are a lot of corpses under the rubble," Yasser Hammo, a civil defence worker, said via an Internet messaging system, adding that volunteers and civil defence workers were still pulling bodies out.

Footage on social media and the pro-opposition Orient TV station showed makeshift ambulances rushing with injured civilians through an area where people were searching for survivors among the debris of collapsed buildings.

One local resident, Sameh al-Muazin, said he had seen mangled bodies in the main Jalaa street of the city, adding that people feared a further round of intensive bombing.

"Everyone is afraid that this is just the beginning," he said.

Idlib, the capital of a northwestern province of the same name, became an important centre for rebel-controlled northwest Syria after it was captured earlier this year by a coalition of Islamist insurgent groups known as Jaish al Fateh which includes al Qaeda's Syrian wing Nusra Front.

Signs of ceasefire collapse 

Russia began a major aerial campaign on September 30 in support of Syrian President Bashar al-Assad, its ally, who earlier in the year suffered a series of setbacks including the loss of Idlib province and other areas of crucial strategic importance.

Moscow says its air strikes target Islamic State militants but rebels and residents say they are causing hundreds of civilian casualties through indiscriminate bombing well away from the front lines.

Residents say they distinguish Russian planes that fly at high altitudes in sorties from Syrian helicopters that mainly drop indiscriminate barrel bombs at much lower heights.

Idlib city been largely spared the intensified aerial bombing campaign witnessed in rural areas after a United Nations-brokered ceasefire deal was reached in September.

The deal allowed for the withdrawal of rebel fighters holed up in a border village near Lebanon in return for the evacuation of civilians from two Shi'ite towns of Kefraya and al-Foua under rebel siege in Idlib province.

The deal included a tacit understanding under which Idlib city also fell under the ceasefire arrangements, allowing thousands of displaced from northern Syria to shelter there.

In a sign the ceasefire had broken, one rebel source said rebels had begun to shell the two towns again. Residents reported tens of families fleeing with some of their belongings to the safety of makeshift camps erected along the Turkish border.

Separately, the Syrian army with the backing of Russian air power said on Sunday it had seized the rebel held town of Khan Touman in southern Aleppo, a major gain that opened the way for advances further to the west in Idlib province.

The advances brought the army only a few kilometres from the major rebel-controlled Aleppo-Damascus highway, whose capture would be a big boost to the Syrian army.