Hundreds of victims appeared with signs of a chemical weapons after an alleged attack in Syria, according to the World Health Organization.

The attack that killed dozens last weekend in Douma, one of the last pockets of rebels against the Bashar Assad regime, has been condemned worldwide by countries such as the U.S., though his supporters including Russia have said there is no evidence that such an attack even occurred.

Details about the attack have been impossible to verify, though the United Nations WHO released its statement Wednesday and said that 70 people were reported dead, with 43 of them stemming from toxic chemicals.

It said an estimated 500 patients showed signs of exposure including irritation of mucous membranes, respiratory failure and disruption to central nervous systems.

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Chemical weapons are banned under international law, though there have been repeated accusations of their use in Syrias bloody and long-running civil war, which has killed more than half a million people.

Last year the U.S. fired cruise missiles at an regime airfield following a chemical attack that Syria and Russia deny but the UN found came from the Assad government.

President Trump has promised a big price to pay for the latest alleged attack, which Israel allegedly responded to by bombing a Syrian base, and observers are watching to see whether he could launch more strikes.

It is unclear how the WHO statement would affect the calculus of a military response, which raises the possibility of Russian and U.S. troops fighting each other in the Middle East.

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The WHO said that it was only conducting an epidemiological investigation rather one that would delve into the circumstances of the use of the weapons.

Russian and U.S.-led proposals for investigations into the matter each fizzled at the UN on Tuesday, though Assad says that he would welcome investigators from the international Organization for the Prohibition of Chemical Weapons.