The United Nations Special Envoy for Syria Staffan de Mistura described a suspected chemical attack that killed at least 58 people, including 11 children, in northwestern Idlib province on Tuesday as “horrific."
The rebel-held town of Khan Sheikhoun was apparently hit by an airstrike that released “toxic gas”.
The US, Britain and France have laid the blame for the attack on the Syrian government but both the Syrian military and ally Russia have denied the use of chemical weapons.
The UN's Commission of Inquiry for Syria said on Tuesday that it had begun investigation into the attack.
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