Syria chemical 'attack': What we know

At least 70 people have been killed in a suspected chemical attack on the rebel-held town of Khan Sheikhoun in north-western Syria. Hundreds suffered symptoms consistent with reaction to a nerve agent after what the opposition and Western powers said was a Syrian government air strike on the area on Tuesday morning. The Syrian military denied using any chemical agents, while its ally Russia said an air strike hit a rebel depot full of chemical munitions. What happened?
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