US President Donald Trump has tweeted that Russia should "get ready" for missiles to be fired at its ally Syria, in response to an alleged chemical attack near Damascus on Saturday.
"Get ready Russia, because they will be coming, nice and new and 'smart!'" Mr Trump said in his tweet.
Senior Russian figures have threatened to meet any US strikes with a response.
President Bashar al-Assad's government denies mounting a chemical attack on the rebel-held town of Douma.
In his tweet, Mr Trump called the Syrian leader a "gas killing animal".
The US, UK and France have agreed to work together and are believed to be preparing for a military strike in response.
What happened in Douma?
Syrian opposition activists, rescue workers and medics say 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".
A team from the Organisation for the Prohibition of Chemical Weapons (OPCW) is due to deploy to Syria "shortly" to determine whether banned weapons were used in Douma.
What is Russia's position?
It has described the reports of the chemical attack as a "provocation" designed to justify Western intervention against its ally.
Several senior Russian figures have warned of a Russian response to a US attack, with Alexander Zasypkin, Moscow's ambassador to Lebanon, repeating on Wednesday a warning by the head of the military that missiles would be shot down and their launch sites targeted if they threatened the lives of Russian personnel.
Also on Wednesday, Russian foreign ministry spokeswoman Maria Zakharova asked whether the aim of Western strikes might be "to quickly remove the traces of the provocation... [so] international inspectors will have nothing to look for in terms of evidence".
Moscow's UN ambassador, Vasily Nebenzia, has warned Washington it will "bear responsibility" for any "illegal military adventure" it carried out.