Similarly, US Secretary of State John Kerry said he now believed there was "strong evidence" of Syria using gas.
The White House previously said it had "varying degrees of confidence" that Syria had used chemical weapons.
Last month, President Barack Obama warned that such a development was a "red line" for possible intervention, but said existent intelligence did not represent sufficient proof.
"This fight is about the terrible choices that the Assad regime has made with its willingness to kill anywhere... to use gas, which we believe there is strong evidence of use of," he said.
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