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 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.
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 happened in Douma?
Opposition activists and rescuers say government aircraft dropped bombs filled with toxic chemicals on Douma.
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, the last major rebel stronghold near the capital Damascus, 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.
Russia also said it would deploy military police to Douma on Thursday and that the situation there had stabilised.
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”.
Addressing new ambassadors in Moscow, President Vladimir Putin said the world was becoming more chaotic. He 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”.
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.
On Wednesday, 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”.
French President Emmanuel Macron said any strikes would “not target allies of the [Syrian] regime or attack anyone, but rather attack the regime’s chemical capabilities”.
But The Times newspaper 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 Sea.
There are reports that Russia has withdrawn battleships from its naval base in the Syrian port city of Tartus:
What is the UN doing?
On Tuesday the UN Security Council failed to approve moves to set up an inquiry into the alleged attack on Douma.
As permanent members of the council, Russia and the US vetoed each other’s proposals to set up independent investigations.
The US-drafted resolution would have allowed investigators to apportion blame for the suspected attack, while Russia’s version would have left that to the Security Council.
The OPCW’s fact-finding mission will not seek to establish who was responsible for the attack.