The foreign secretary says both the British and US governments have seen evidence that the regime in Syria is preparing to use chemical weapons.
The BBC's security correspondent Frank Gardner spoke to Mr Hague. He said: "We can't be specific but we have seen enough evidence to know that they need a warning. The president of the United States has talked about "serious consequences" and clearly he means it and so we meant it."
Gardner told Today's presenter John Humphrys: "There are two risks. One, is that chemical weapons are used against their own people. And the other risk is that they fall into the wrong hands given the large number of Jihadists operating in the area."
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