Victims of a suspected chemical attack in Syria on Tuesday appeared to show symptoms consistent with reaction to a nerve agent, the World Health Organization said on Wednesday.

"Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," WHO said in a statement, putting the death toll at least 70.

"The likelihood of exposure to a chemical attack is amplified by an apparent lack of external injuries reported in cases showing a rapid onset of similar symptoms, including acute respiratory distress as the main cause of death," it said.