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.
More top news
Big companies will have to show that their supply chains do not use child slave labour or exploit people, David Cameron has announced.
Disneyland Paris is reportedly facing a pricing probe amid claims it charges British tourists more than its French customers.
Overworked GPs are becoming so tired they could be putting patient's safety at risk, the head of British general practitioners has warned.