This led Mr Trump to take to social media, tweeting: "Russia vows to shoot down any and all missiles fired at Syria".

It consistently says the crisis needs a "political solution" but has numerous times vetoed UN Security Council measures aimed at addressing the conflict - including an investigation of war crimes in the country.

Russia later hit back, with Kremlin spokesman Dmitry Peskov saying "we do not participate in Twitter diplomacy" in comments reported by the Interfax news agency. He suggested, "stop the arms race?"

He too called for a parliamentary vote on any UK involvement in military action, saying: "The Government must present the objectives of any proposed action to Parliament". Some of the victims were discovered with foam coming out of their mouths, huddled in homes that reeked of chlorine.

He also said, "There is no reason for this". Moscow said it has warned about false-flag attacks being prepared in recent months.

He had repeatedly said he would not telegraph military moves towards foes.

When the Syrian government went ahead and did it anyway, prompting no response from Obama, Syrian leader Bashar al-Assad was filled with impunity. "Could be very soon or not so soon at all!"

The United States was actively collecting evidence on how Syria and Russia were responding to US threats, officials said, adding that Washington believed Syria and its allies were almost certainly also moving personnel and military equipment - beyond just aircraft - into protective locations.

Aid groups have blamed the Assad regime, backed by Russia and Iran.

The decision came after an apparent chemical weapons attack on civilians in Syria.

The Violations Documentation Centre, which records alleged violations of international law in Syria, said bodies were found with foam at the mouth, discoloured skin and burns to the eyes.

French President Emmanuel Macron, who has been coordinating closely with Washington, said he would decide on a response "in the coming days".

Trump wrote in an incendiary early morning tweet that the missiles would be "nice and new and smart!", after a Russian official said any strike in retaliation to the attack on the rebel-held town of Douma would be shot down.

The World Health Organization said about 500 patients showed signs of exposure to toxic chemicals following the Douma attack.

The FBI seized a number of materials from Trump's longtime lawyer Michael Cohen on Monday, after prosecutors in New York received a referral from Mueller's team, Cohen's attorney said.

The Daily Mail says that Britain should support the US "in all but the most exceptional circumstances", although the case case for joining strikes has not yet been made.

Amid the unprecedented tension between the two former Cold War adversaries, the Syrian Observatory for Human Rights said the Syrian government evacuated forces from a number of air bases around the country. "I don't know for sure, but I know it would embolden others if this goes unchecked".