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 had promised a "forceful" reply to the suspected attack.
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.
After the attack, Mr Trump and his Defence Secretary, James Mattis, cancelled travel plans.
The US and Western allies said they have agreed to work together.
But several senior Russian figures 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.
Mr Trump's tweet referred to this warning.
Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and “smart!” You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!
The US and allies - France and Britain - are believed to be preparing for a military strike.
US President Donald Trump cancelled his first official trip to Latin America so he could focus on Syria.
That decision suggests the US response may involve a larger military operation than a limited strike, says the BBC's Barbara Plett Usher in Washington.
French President Emmanuel Macron said any strikes would target Syrian government chemical facilities.
But The Times reports that the UK's Prime Minister Theresa May has urged Mr Trump to provide more evidence of the suspected chemical attack.
A US Navy guided-missile destroyer, the USS Donald Cook, is in the Mediterranean, and the European air traffic control agency, Eurocontrol, has warned airlines to take "due consideration" while in the eastern Mediterranean over the next few days, because of the possible launch of missiles into Syria.