The "Gas Killing Animal" appeared to be a reference to Assad, whose forces allegedly used chemical weapons against dissident civilian population in Syria. Another option - strike the locations where chemical weapons are stored.
Trump's threats of retaliatory military action prompted a slew of Russian comments that warned that United States strikes could trigger direct military clash between the two former Cold War adversaries.
He said: "We hope that there will be no point of no return - that the USA and their allies will refrain from military action against a sovereign state".
President Vladimir Putin's spokesman Dmitry Peskov said in televised remarks that "we don't participate in Twitter diplomacy". We are proponents of a serious approach.
He said: "Well I think the likely outcome is that action will be taken because it must be taken".
To which Maria Zakharova, the spokesperson of the Russian Foreign Ministry, shot back on her Facebook page: "Great idea".
Tass quoted Zakharova as saying, "Smart missiles must strike terrorists, not the legitimate government that has been fighting global terrorism on its territory for several years".
The US relationship with Russia is worse than the Cold War era, President Donald Trump said today and blamed special investigator Robert Muellers "fake and corrupt" investigations into the Russian meddling in 2016 election for the "bad blood" between the two countries.
Russia's Ambassador to Lebanon, Alexander Zasypkin, warned Tuesday evening in an interview on Hezbollah-linked Al-Manar TV that any USA missiles fired at Syria will be shot down and their launch sites targeted.
Trump had tweeted earlier that Russian Federation shouldn't be allied with a "Gas Killing Animal", a reference to Syrian President Bashar Assad, whose regime is accused of killing dozens of people in a suspected chemical attack on rebel-held Douma over the weekend.
Trump is consulting with worldwide allies and military advisers about a strike on Syria. But he has threatened military action in response to Syria's suspected chemical attack, which activists and rescuers say killed at least 40 people.
Before his White House trip, Mattis told reporters that the U.S.is still assessing intelligence as to whether the Assad regime is responsible for the gas attack that killed anti-government rebels and their children.
Officials of the Trump administration have been discussing with officials of France and Britain a possible joint military response to Syria's alleged poison gas attack.