It's a nightmare scenario for Syria: The United States launches strikes on Syrian targets, killing Russians, and Moscow is forced to defend itself or retaliate.
Moscow took the step after the US, France, Germany and United Kingdom held consultations on launching a military strike as early as the end of this week.
Donald Trump and Western allies are in talks over a possible military strike against Syria.
But the pinpoint strike did not deter Assad and U.S. officials, speaking on condition of anonymity, have since investigated as many as 10 suspected chemical attacks.
The Russian leader warned against "ill-considered and risky actions. that would have consequences beyond conjecture".
Earlier in the day, Yevgeny Serebrennikov, first deputy chairman of the Russian upper house's defense committee, had warned that Russia will respond immediately if its servicemen in Syria get hit by potential USA strikes.
He wrote Wednesday: 'Our relationship with Russian Federation is worse now than it has ever been, and that includes the Cold War.
At least 60 people were killed and more than 1,000 injured in Saturday's suspected attack on Douma, then still occupied by rebel forces, according to a Syrian relief group.
US President Donald Trump emerged from a meeting with his national security team on Thursday without a "final decision" on how to respond to the suspected chemical attack.
The Eurocontrol airspace organization said that the European Aviation Safety Agency had sent a "Rapid Alert Notification" that flight operations needed to consider the possibility of air or missile strikes into Syria. The BBC reported that Mrs May was ready to give the go-ahead for Britain to take part.
Several Russian observers said they believed Russia may try to shoot down some USA missiles as a token symbol of resistance.
Media reports suggested that Trump and his advisers had been weighing a "powerful" military response to the alleged use of a chlorine bomb, deeming it the only credible way to deter further chemical attacks.
The State Department said the symptoms reported were "consistent with an asphyxiation agent and of a nerve agent of some type" and investigators are headed to the scene.
"It's some of the worst piece of fake news we've seen yet from the Russian Federation propaganda machine", Pierce told reporters at United Nations headquarters in NY.
Alexander Sherin, deputy head of the State Duma's defense committee, said Trump "can be called Adolf Hitler No. 2 of our time - because, you see, he even chose the time that Hitler attacked the Soviet Union".
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"The Assad regime's use of chemical weapons that led to the murder of dozens of Syrians - many of them children - is barbaric and a clear violation of worldwide law", said Heller. After months of criticism that Trump was being soft on Putin, the US president pulled no punches with a Tweet on Sunday: "President Putin, Russia and Iran are responsible for backing Animal Assad".
With Russian military assistance, President Assad launched a bloody offensive on Eastern Ghouta, which had been under rebel control since mid-2013.
The American president also criticized Russian Federation for its support of Syrian leader Bashar Assad, saying "you shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!'"
Some lawmakers criticized Trump's apparent telegraphing by tweet of a missile strike in Syria.
On Wednesday, the president tweeted that U.S. missiles "will be coming" and told Russian Federation, which has forces in Syria, to "get ready".
Russia has previously threatened to shoot down USA missiles and aircraft that threaten the lives of Russian personnel in Syria, but since the crisis began it has been largely silent on how it will respond. We are totally predictable. He said any big tragedy can occur in the world any time.
Whether he is bluffing or double-bluffing, it seems that a strike is imminent.
Here's who Russian Federation has blamed this week for the alleged chemical attack that killed dozens, including many children.
At a meeting on Monday, U.S. Ambassador to the United Nations Nikki Haley said Washington would respond to the suspected attack whether the Security Council acted or not.
The tensions within Syria have the potential to explode into an worldwide conflict at any moment.
Assad said Thursday that Western threats to strike Syria are based on "lies" and seek to undermine his forces' advances near Damascus.
USA officials have consulted with global allies on a possible joint military response to Syria's alleged poison gas attack on a rebel-held town.
Meanwhile, the Russian Navy is about to start exercises near the coast of Syria - something surmised from an worldwide warning issued to flight personnel (NOTAM) and another issued for sailors, Russian news agency Interfax is reporting.
However, US forces in February killed or injured hundreds of Russian contractors fighting on Assad's side during a confrontation in Deir al-Zor province. In June 2017, a USA jet shot down a Syrian warplane that was trying to bomb American-backed fighters.