The opposition says the evacuation deal, brokered by Iran and Qatar, leads to forced demographic change.
America's envoy to the U.N. rejected Russia's claim that the gas may have accidently leaked from rebel depot.
In a separate development, over 58 people were killed in a chemical attack in Idlib province on Tuesday.
The U.S. condemns the “outrageous and indefensible choice” that shields Syria's use of chemical weapons.
The Syrian regime drew sharp criticism for bombing the civilian-packed city in the final stage of the campaign.
Accusing the Syrian government of committing “many and large” ceasefire violations, the rebel groups on Monday threatened to boycott the peace conference scheduled in Astana, Kazakhstan for later this month.
Peace talks to be held in Kazakhstan within a month if truce survives.
At least 26 people, most of them children, were killed after Russian planes bombarded a residential area and a school complex in the province of Idlib in northwest Syria on Wednesday.
Rebel groups reject proposal calling it a plan to isolate & target them.
E.U. to push for strong condemnation of Moscow's role in Syria conflict.
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