"As we have said, Assad has no legitimacy to lead the Syrian people," he added, in a statement to reporters.
Kirby said Washington and its allies want a "political transition away from Assad that brings an end to such attacks and leads to a future that fulfils Syrians' aspirations for freedom and dignity."
Sunday's raids on the town of Douma, in the rebel bastion of Eastern Ghouta, marked one of the bloodiest regime onslaughts in Syria's four-year war.
And they came two years after devastating chemical weapons attacks on the same region that much of the international community blamed on the Syrian government.
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Kirby said Washington and its allies want a "political transition away from Assad that brings an end to such attacks and leads to a future that fulfils Syrians' aspirations for freedom and dignity."
Sunday's raids on the town of Douma, in the rebel bastion of Eastern Ghouta, marked one of the bloodiest regime onslaughts in Syria's four-year war.
And they came two years after devastating chemical weapons attacks on the same region that much of the international community blamed on the Syrian government.
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