US President Donald Trump has warned Russia of imminent military action in Syria over a suspected poison gas attack, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar al-Assad.
Mr Trump was reacting to a warning from Russia on Tuesday that any US missiles fired at Syria over the deadly assault on a rebel enclave would be shot down and the launch sites targeted.
His comments raised fears of direct conflict over Syria for the first time between the two world powers backing opposing sides in the country's protracted civil war, which has aggravated instability across the Middle East.
"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!" Mr Trump tweeted, referring to Moscow's alliance with Assad.
In response, Russia's Foreign Ministry said in a Facebook post that "smart missiles should fly towards terrorists, not towards the lawful government".
Russia later hit back, Kremlin spokesman Dmitry Peskov saying "we do not participate in Twitter diplomacy" in comments reported by the Interfax news agency.
Foreign Ministry spokeswoman Maria Zakharova said any US missile salvo could be an attempt to destroy evidence of the reported gas attack in the Syrian town of Douma, for which Damascus and Moscow have denied any responsibility.
Dozens of Douma inhabitants died and hundreds were injured in the attack, according to the World Health Organization (WHO).
After Mr Trump's tweet, the Syrian Observatory for Human Rights - a British-based war monitor with a network of sources on the ground - reported that pro-government forces were emptying main airports and military air bases.
The Russian military said later it had observed movements of US naval forces in the Gulf. Any US strike would probably involve the navy in waters within range of Syria, given the risk to aircraft from Russian and Syrian air defence systems. A US Navy guided-missile destroyer, the USS Donald Cook, is in the Mediterranean.
The Syrian foreign ministry accused the United States, which has supported some rebel groups in Syria's conflict, of using "fabrications and lies" as an excuse to hit its territory.
"We are not surprised by such a thoughtless escalation by a regime like the United States regime, which sponsored terrorism in Syria and still does," the state news agency SANA cited an official source in the ministry as saying.
In London, British Prime Minister Theresa May said all the indications pointed to Syrian government responsibility for the Douma attack and such "a shocking and barbaric act" could not go unchallenged.
The World Health Organization said on Wednesday that 43 people had died in Saturday's attack on Douma from "symptoms consistent with exposure to highly toxic chemicals", and more than 500 in all had been treated.
With tensions growing pan-European air traffic control agency Eurocontrol warned airlines to exercise caution in the eastern Mediterranean due to the possible launch of air strikes into Syria over the next 72 hours.