Shelling of a rebel-held area in the northeast of the Syrian capital killed 13 children Wednesday, a monitor said revising an earlier toll, with activists saying a school had been hit.

"The number of children... killed by mortar fire on areas of Qabun district has risen to 13," said the Syrian Observatory for Human Rights, adding: "The toll might rise because several others were critically injured."

Activists blamed forces loyal to President Bashar al-Assad for the killings, which they described as a "massacre of children".

The Syrian Revolution General Commission, a network of activists across the war-torn country, said the shelling hit a school, and an activist group in the neighbourhood shared a grisly video of the aftermath.

The footage posted on the group's Facebook page showed the bodies of at least nine children lying on a white tiled floor, some of them surrounded by smears of blood.

Rebels and the army reached a truce in Qabun earlier this year, but activists say there have been frequent violations.