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 near Damascus at the weekend.
Senior Russian figures had threatened to meet any US strikes with a response.
Mr Trump had promised a "forceful" reply to the suspected attack.
President Bashar al-Assad's government, which is backed militarily by Russia, denies being behind the chemical attack on the rebel-held town of Douma.
In his tweet, Mr Trump called Mr Assad a "gas killing animal".
On Saturday, Syrian opposition activists, rescue workers and medics said government aircraft dropped bombs filled with toxic chemicals on Douma.
The Syrian-American Medical Society (SAMS), which supports hospitals in rebel-held areas, and rescue workers the Syria Civil Defence said more than 500 people were treated for symptoms "indicative of exposure to a chemical agent".
On Wednesday, the UN's World Health Organization demanded access to the area to verify reports from its partners, who include Sams, that 70 people had died - including 43 who showed "symptoms consistent with exposure to highly toxic chemicals".
The US, UK and France have agreed to work together and are believed to be preparing for a military strike in response, reports BBC.
