Dozens of people, including civilians, have been killed in air strikes on the rebel-held Syrian city of Idlib, according to several sources, in some of the most ferocious raids there in months.

The Syrian Observatory for Human Rights said on Tuesday that at least 26 people were killed and casualties were expected to rise as rescue workers searched for bodies under the rubble.

"Ten civilians, mostly women, are among the dead," Rami Abdel Rahman, head of the UK-based monitoring group tracking development's in Syria's conflict, told AFP news agency.

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Abdel Rahman said the raids were probably carried out by Russian fighter jets - allied with Syria's government - or by a US-backed air coalition.

The strikes also wounded scores of people and levelled several multi-storey buildings in residential areas of the northwestern city, according to witnesses, who told Reuters news agency that the extent of the damage and the debris bore the hallmarks of a Russian attack.

But Russia's defence ministry said later on Tuesday media reports that its planes had bombed Idlib were not true, Interfax news agency reported.

The Civil Defence, a volunteer rescue group operating in rebel-held areas, gave a different death toll, saying 15 bodies were pulled from the rubble and that 30 wounded people were taken for treatment.

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Video footage by activists on social media showed civilians, including young children, being treated in a main city hospital where the injured had been taken for treatment.

"We are still pulling bodies from the rubble," Issam al Idlibi, a volunteer civil defence worker, told Reuters.

Russian planes have targeted a number of towns and villages in the area since entering the Syrian conflict in September 2015 to back ally President Bashar al-Assad.

Activists and residents said there had been a reduction of Russian strikes in Idlib province since a Turkish-Russian brokered cessation of hostilities late December.

Planes from the US-led coalition have launched a number of attacks in the rural province, a major stronghold of fighters, many of them formerly affiliated to al-Qaeda.

Idlib's population has been swollen by thousands of Syrian fighters and their families evacuated from villages and towns around Damascus and Aleppo city, which was retaken by the government in recent months.