"You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"

Soon after the tweets, Russia's foreign ministry said that "smart missiles should fly towards terrorists, not legal government".

With punitive USA military action seemingly imminent, Russian Federation scrambled to deflect blame from its ally Bashar al-Assad and, according to a monitor group, Syrian forces evacuated key defense buildings in Damascus.

US Defense Secretary Jim Mattis, striking a cautious tone hours after Trump's threat of missile strikes, said the US was assessing intelligence about the suspected attack.

"We stand ready to provide military options if they're appropriate, as the president determined", he said.

The decision to order military action rests in the hands of a serving Prime Minister, although convention dictates that MPs and peers are kept up to date with developments.

The official, Lt. Gen. Viktor Poznikhir, deputy chief of operations of the Russian General Staff, said the United States should instead pay attention to the humanitarian situation in the city of Raqqa, which Kurdish-led forces and their American allies captured from the Islamic State past year. Indeed, the Assad regime would not have carried out the attack in Douma if it didn't think it could handle another barrage of cruise missiles.

In an earlier Wednesday tweet, Trump said that there was "So much Fake News about what is going on in the White House".

The Russian military said it had observed movements of US Navy forces in the Gulf.

He suggested that the American rockets would evade Russian air defences because they're "nice and new and 'smart'". The destroyers were underway in the Mediterranean Sea when the missiles were launched from hundreds of miles away.

A child is treated in a hospital in Douma, eastern Ghouta in Syria, after what a Syria medical relief group claims was a suspected chemical attack April, 7, 2018.

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In recent weeks, government forces have recaptured villages and towns in the eastern Ghouta suburbs of the capital. The toll is likely to rise, said the Union of Medical Care Organizations (UOSSM), a group of global aid agencies.

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"We have become accustomed to such threats that aim to frighten the Syrian people", said Marwan Ghata, 66, an engineer.

The constitutional grey area over the need for parliamentary approval for military action has led to repeated calls for reform.

The reaction from Assad backer Moscow is unpredictable and Russian Federation has threatened retaliatory action against the United States if missiles are fired at Syria.

The officials declined further comment and it was not clear whether the Syrian moves would impact United States military planning for potential action against Syria over a suspected poison gas attack.

"We don't comment on submarine movements", a spokesman said.

British Prime Minister Theresa May has summoned her cabinet to brief ministers this Thursday on a "fast-moving week" amid the ratcheting up of tensions over Syria. The strike a year ago made good on Trump's vow not to let the use of chemical weapons go unpunished, but it failed to deter Assad from using them again. Global inspectors are seeking clearance from Damascus to visit Douma under safe conditions to determine whether globally banned munitions were used, though they will not assign blame.

"The continued use of chemical weapons can not go unchallenged".

French President Emmanuel Macron on Tuesday called for a "strong and joint response" to the attack in the Syrian town of Douma on Saturday. In the aftermath of the attack, he criticized Russian President Vladimir Putin and Iran for supporting Syrian President Bashar al-Assad and threatened there would be a "big price to pay" for the deaths.

Trump's tweet also drew a quick response from Russia's Foreign Ministry.

The Eurocontrol airspace organization said that the European Aviation Safety Agency had sent a "Rapid Alert Notification" that flight operators needed to consider the possibility of air or missile strikes on Syria.