The White House has said it is "highly likely" that it is President Bashar al-Assad's regime, not rebels, behind any chemical weapons use.
Earlier, a leading member of the Independent International Commission of Inquiry on Syria, told Swiss TV statements from victims of the conflict suggested rebels had used the nerve agent Sarin.
Carla Del Ponte said that there was "strong, concrete suspicions but not yet incontrovertible proof".
However, she did not rule out the possibility that they might also have been used by government forces.
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