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"If this were proved to have been committed by the Assad regime then it would be another reason to think they are an absolutely heinous outfit, it is a war crime," Johnson told reporters at a news conference in London.
"Bombing your own civilians with chemical weapons is unquestionably a war crime and they must be held to account."
The suspected Syrian government chemical attack killed at least 58 people, including 11 children, but a Syrian military source denied the army had used any such weapons.