Syrian primary schoolgirls held a graduation ceremony in honour of their "high-achieving" classmate on Wednesday, after she was killed in a Russian airstrike a day earlier.

The tearful year-six students at the Idlib-based school showcased certificates for 12-year-old Hanan Mousa who was killed along with 18 other children in the early-morning strike on a residential neighbourhood.

Hanan was very well-mannered and an excellent student who was impatiently looking forward to receiving her certificate, one of her classmates told The New Arab.

On Tuesday morning, Russian airstrikes killed at least 26 people in the rebel-held Syrian city of Idlib, in what is understood to be one of the heaviest raids there in months.

Around ten attacks wounded scores of people and crushed several multi-storey buildings in residential areas of the northwestern city, the Syrian Observatory for Human Rights reported on Tuesday.

"We are still pulling bodies from the rubble," Issam al Idlibi, a volunteer civil defence worker, told Reuters on the day of the attack. Most of the casualties were civilians and the death toll would probably rise, he added.

The extent of the damage and the debris bore the hallmarks of a Russian attack, said two witnesses, although was no immediate comment from Moscow.

Planes from the US-led coalition, as well as Russian and Syrian forces, have launched a number of attacks in the rural province, a major stronghold of Fateh al-Sham rebels, formerly affiliated to al-Qaeda and not party to the Russia-Turkey brokered ceasefire that went into effect on 30 December.

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