The UN's humanitarian chief fiercely condemned attacks against civilians in Syria at a press conference in Damascus on Monday, a day after one of the bloodiest government raids in the four-year war.

"I am horrified by the total disrespect for civilian life in this conflict," Stephen O'Brien said in a statement.

He said he was "particularly appalled" by reports of civilian deaths in Sunday's air strikes on the rebel-held town of Douma, and said attacks on civilians "must stop".