Donald Trump has warned that military action is imminent in Syria, declaring missiles “will be coming”.
Bashar Assad’s regime has called the threats “reckless” and Moscow said air strikes would destroy evidence of a chemical attack.
The US president has been in talks with Britain and France to secure a joint response to the suspected chemical weapons attack in the rebel-held town of Douma.
In a tweet responding to the threat, President Trump said: “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!”
Soon afterwards, however, he suggested it was time to “stop the arms race” and said there was no reason for the low in relations between Russia and the US.
He tweeted: “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?”
Russia’s ambassador to Lebanon, Alexander Zasypkinhad, warned that the federation’s military will shoot down US rockets and “even the sources that launched the missiles”.
Mr Trump’s decision to announce in highly inflammatory language on Twitter that missile strikes are imminent, rather than retaining an element of surprise, will raise eyebrows among military chiefs.
Soon after the tweets, Russia’s Foreign Ministry said that “smart missiles should fly towards terrorists, not legal government”.
Russian Foreign Ministry spokeswoman Maria Zakharova questioned whether the idea behind military action was to “use the smart missiles to sweep the traces of the provocation under the rug”.
Russia vetoed a draft resolution at the United Nations which sought to create a new body to determine responsibility for the attack.
Theresa May agreed with President Trump and French President Emmanuel Macron on Tuesday that the international community needed to respond to the attack.
The Prime Minister has repeatedly refused so far to be drawn on whether the UK would be involved in any military response.
She faces pressure to secure support from the Commons, which is currently in recess for Easter, for involving Britain in air strikes but has the power to take the decision without the backing of MPs.
General Sir Richard Barrons, a former commander of Joint Forces Command, said Russia’s warnings that launch platforms could be targeted in response to air strikes meant “they are going to try and sink ships, sink submarines and shoot aircraft out of the sky – that’s war”.
Former soldier Johnny Mercer, who sits on the Commons Defence Select Committee, said Britain played a calming role as he criticised the “unhelpful rhetoric” tweeted by the US president.
The attack in Douma happened late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
Syrian opposition activists and rescuers said poison gas was used on the rebel-held town near the capital, an allegation strongly denied by the Assad government.
Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.
Reports suggested that more than 500 people, mostly women and children, were taken to medical centres with breathing problems, foaming at the mouth and burning sensations in the eyes.
The attack comes almost exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base.