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The delay is the result of last-minute rebel demands for the release of prisoners held in government prisons.
Medical sources in Khan Sheikhun reported symptoms among the affected including fainting, vomiting and foaming at the mouth.
A Russian military helicopter is shot down by ground forces, killing five people over Idlib.
NATO has warned Russia to keep its aircraft, currently in Syria, out of Turkey's air space.
Syrian opposition fighters have ousted government troops from a major airbase after a two-year battle in the province of Idlib.
The U.S. is involved in Syria: An American citizen-turned-suicide bomber for Al Qaeda attacked a Syrian government complex on Sunday .