Donald Trump has told Russia and Syria to "get ready" for a missile attack on the Assad regime, saying the bombs will be "nice and new and smart".
Mr Trump tweeted an extraordinary response to Russia's claim that it would shoot down any missiles fired at Syria following the chemical weapons attack on Douma.
The US President added: "You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"
Mr Trump described the relationship between the US and Russia as "worse now than it has ever been, and that includes the Cold War" but insisted there was "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," he said.
"Stop the arms race?"
Earlier, Alexander Zasypkin, the Russian envoy to Beirut, said: “If there is an American strike, then we... will shoot down the missiles and target the positions from where they were launched.
“In the past few days, we have seen an escalation towards a significant crisis."
Mr Zasypkin’s comments, made in an interview with a Hizbollah-affiliated television channel, are the sternest Russian warning yet against American strikes. They go beyond previous threats that Russian troops would use their missile defence systems to shield the Assad regime.
Mr Trump's tweets increase the pressure on Theresa May, the Prime Minister, to give her unequivocal backing to air strikes against targets in Syria, having said on Tuesday that she and Mr Trump would not "allow the use of chemical weapons to continue".
On Wednesday Moscow also suggested US plans to strike could be a pretext to destroy evidence of the alleged chemical weapons attack, which Russia has said was a staged "provocation" to justify Western intervention.
"Is the whole idea to quickly remove the traces of the provocation...(so) the international inspectors will have nothing to look for in terms of evidence?" asked foreign ministry spokeswoman Maria Zakharova in a Facebook post.
Syria described Mr Trump's threats as a "reckless escalation". "We are not surprised by such a reckless escalation from a regime like the United States which has fostered and continues to foster terrorism in Syria," an official source at the foreign ministry said.
Earlier in the day Eurocontrol, the pan-European air traffic control agency, warned civilian airliners to be cautious above the eastern Mediterranean because of the possibility of strikes in the next 72 hours.
The agency warned that both missiles launched from aircraft or from naval forces might be launched before the end of the week.
"Due to the possible launch of air strikes into Syria with air-to-ground and/or cruise missiles within the next 72 hours, and the possibility of intermittent disruption of radio navigation equipment, due consideration needs to be taken when planning flight operations in the Eastern Mediterranean,” the agency said in a statement.
The US has two Navy destroyers capable of firing a barrage of cruise missiles in the eastern Mediterranean already. An American naval strike group, led by the aircraft carrier USS Harry S Truman, is moving towards the area.
The World Health Organisation said that 500 people were treated in last weekend’s suspected chemical weapons attack in Douma and demanded access to the site.
The global health agency said it estimated 500 were brought to hospital with "signs and symptoms consistent with exposure to toxic chemicals”.
"WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts, and to deliver a comprehensive public health response," said Peter Salama, the deputy director-general for emergency preparedness and response.
The Syrian regime has so far prevented UN agencies from visiting the site of the alleged attack, although Russian military inspectors have been allowed access.
Meanwhile, Theresa May has given her strongest signal yet that Britain would support President Donald Trump in military action against the Syrian regime as the two leaders resolved “not to allow the use of chemical weapons to continue”.
The Prime Minister spoke to both Mr Trump and the French President Emmanuel Macron by telephone during which all three agreed that Syrian President Bashar al-Assad had shown “total disregard” for international laws against the use of such weapons.
A Trump official upped the diplomatic tension by describing the chemical attack on Douma, Syria, as “genocide” and saying a military response was “appropriate”.