Amateur video posted on YouTube by activists showed horrific images of the children's corpses and of a man holding the inanimate body of a little girl in a light blue dress who had been decapitated by the force of the strike.

"Allahu akbar! (God is greatest)" mourners are heard to cry in the footage. "These are the reforms of (President) Bashar al-Assad!"

"Only God can help us! The whole of humanity has failed us! Nobody has helped us!" cries another.

According to Observatory director Rami Abdel Rahman, the attack came a day after five other children were killed in Saraqeb, also in Idlib province.

"This is the gift of the regime to the children of Syria on their first day back at school," he told AFP by telephone.

"The shelling of Kafr Aweid, like all the regime's massacres, is taking place while the whole world just watches."

More than 27,000 people have been killed since the uprising against Assad's rule erupted in March last year, the Britain-based Observatory estimates. The United Nations puts the toll at 20,000.