(CNN) About 500 people may have been affected by a suspected chemical attack last week on Syria's rebel-held town of Douma, according to the World Health Organization.

It remains difficult to place an exact figure on the number of people killed and wounded Saturday on the outskirts of Damascus, a development that has pushed the United States and its allies to the verge of military confrontation in Syria.

But in a statement Wednesday, WHO, citing reports from its health partners, said an estimated 500 patients seen at medical facilities exhibited symptoms consistent with exposure to toxic chemicals.

These symptoms included respiratory failure, severe irritation of mucous membranes and disruption to the central nervous system.

WHO said more than 70 people taking shelter in basements reportedly died in the attack, with 43 of those deaths related to exposure to toxic chemicals.

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