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 one of his tweets on Wednesday, Mr Trump called the Syrian leader a "gas killing animal".
In another, he painted a dark picture of US-Russia relations but said it did not have to be that way.
Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race?— Donald J. Trump (@realDonaldTrump) April 11, 2018
The US, UK and France have agreed to work together and are believed to be preparing for a military strike in response to the alleged chemical attack at the weekend.
What happens next?
On Tuesday, Mr 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.
But following President Trump's early morning tweets on Wednesday, other administration figures struck a more circumspect note.
Defence Secretary James Mattis said the US was still assessing the chemical attack and that the US military stood ready "to provide military options if they are appropriate as the president determines".
White House spokeswoman Sarah Sanders said a final decision had not been made, and "all options remain on the table".
French President Emmanuel Macron said any strikes would target "the regime's chemical capabilities".
In the UK, sources told the BBC that Prime Minister Theresa May looked ready to join military action in Syria without seeking parliamentary consent first.
A US Navy guided-missile destroyer, the USS Donald Cook, is in the Mediterranean Sea.
There are reports that Russian battleships have left the country's naval base in the Syrian port city of Tartus:
ISI reveals: Disappearance of most of the #Russian #Naval Forces from #Tartus Port, #Syria.— imagesatintl (@imagesatint) April 11, 2018
Those missing naval vessels have now been deployed at sea due to possible near-future #strikes. Only one #kilo class submarine remained.#russiannavy #Syriastrikes #foxnews #kilo pic.twitter.com/guRA9w0qqt
What happened in Douma?
Opposition activists and rescuers say government aircraft dropped bombs filled with toxic chemicals on the town, the last major rebel stronghold near the capital Damascus.
The Syrian-American Medical Society (Sams), which operates in rebel-held areas, and local aid workers said more than 500 people had been treated for symptoms "indicative of exposure to a chemical agent".
On Wednesday, the UN's World Health Organization demanded access to verify reports from its partners, which 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.
Douma was under renewed assault from Syrian and Russian forces last week.
Rebels have now been evacuating the town under an agreement involving the Russian military.
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.
It said on Wednesday that samples taken from the site had not revealed any chemical substances.
Senior Russian figures have warned of a Russian response to a US attack. On Wednesday Alexander Zasypkin, Moscow's ambassador to Lebanon, repeated 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".
Addressing new ambassadors in Moscow, President Vladimir Putin said he hoped common sense would prevail and that the situation would stabilise.
Mr Putin said Russia would "keep all its international obligations in full".