The statement was the latest in a war of words raging between Washington and Moscow since Saturday's suspected chemical attack on the rebel-held town Douma, just outside Damascus, which killed more than 40 people according to activists and first responders.
Earlier in the day, Yevgeny Serebrennikov, first deputy chairman of the Russian upper house's Defense Committee, has warned that Russia will respond immediately if its servicemen in Syria get hit by potential United States strikes.
"You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it", he wrote, a reference to Syrian President Bashar al-Assad.
Trump further chided Russian Federation for its support of Syrian President Bashar al-Assad in Wednesday's tweet, saying it "shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" There is no reason for this.
"If there is a strike by the Americans, the missiles will be downed", said Alexander Zasypkin, Russia's ambassador to Lebanon, on Tuesday evening. Meanwhile, President Donald Trump said this attack will be "met forcefully".
The UK is gearing up to take military action against Syria - and it could come as soon as Thursday. Trump has largely avoided criticizing Russian President Vladimir Putin by name, though he singled him out in a tweet over the weekend for supporting Assad.
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"More than 70 people sheltering in basements have reportedly died, with 43 of those deaths related to symptoms consistent with exposure to highly toxic chemicals", the organization added.
Russian Foreign Ministry spokeswoman Maria Zakharova responded to Trump's tweet saying, "Smart missiles should fly towards terrorists, not [Syria's] lawful government, which has spent several years fighting against global terrorism on its territory".
"As the President noted on April 8, the chemical weapons attack by the Syrian regime against innocent civilians in Douma, Syria on April 7 was horrifying, and demands an immediate response from the worldwide community".
The United States and its allies were quick to blame the attack on Damascus, emphasizing that a "history" of using such weapons by the Syrian authorities was "not in dispute".
It also reacted to a warning of imminent military action in Syria from U.S. President Donald Trump. "I am not like other administrations, where they say, 'We are going to do this in four weeks.' It doesn't work that way".
Without naming Syria and its close ally Russian Federation - who have both denied that chemical weapons were used at Douma - the World Health Organization reminded "parties to the conflict" that there is an outright ban on the use of chemical weapons under worldwide law. "Whenever we propose anything meaningful on Syria, Russia vetoes it". He said any military strike would be illegal because it hasn't been authorized by Congress.
Trump did not detail what a strike on Syria would look like, or whether these would be USA missiles.