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”.
On Saturday, Syrian opposition activists, rescue workers and medics said the rebel-held town of Douma in the Eastern Ghouta region had been attacked by government forces using bombs filled with toxic chemicals.
The Syrian-American Medical Society said more than 500 people had been found with symptoms “indicative of exposure to a chemical agent”, and on Wednesday the World Health Organization demanded access to the area to verify reports from its partners that 70 people had died.