US President Donald Trump has tweeted that Russia should "get ready" for missiles to be fired into Syria, in response to an alleged chemical attack at the weekend.
Senior Russian figures had threatened to meet any US strikes with a response.
Mr Trump previously promised a "forceful" response.
President Bashar al-Assad's government - which receives military backing from Russia - denies being behind any chemical attack.
In his tweet, Mr Trump called Mr Assad a "gas killing animal".
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