Trump unleashed his counter-threat hours after Russia's ambassador to Lebanon warned that his country will shoot down any US missiles fired at Syria in a punitive strike on the Damascus regime over its alleged use of chemical weapons.
It was not immediately clear whether the announcement would delay or prevent a USA strike in Syria.
Boris Johnson has criticised Russian Federation after it vetoed an American-drafted resolution at the United Nations to create a new body to determine responsibility for the suspected Syria chemical weapons attack.
Vladimir Shamanov, a retired general who heads the defense affairs committee in the lower house of parliament, said in televised remarks Tuesday that a US strike in Syria could hurt Russian servicemen and trigger retaliation.
Global chemical weapons experts are expected to go to Douma to investigate the suspected poison gas attack.
Images of gasping children struggling to breathe shocked the world and galvanized President Donald Trump, who condemned the attack as "sick" and attacked Russian President Vladimir Putin and Iran for supporting Syrian President Bashar al-Assad. He's often said he doesn't like to broadcast in advance what he would do, in terms of military strikes.
The Syrian air base was struck by missiles a little more than 24 hours after the alleged chemical attack. And the White House says that in a call between Trump and U.K. Prime Minister Theresa May, the leaders "agreed not to allow the use of chemical weapons to continue". The attack would probably be created to send a message without destroying the Assad regime.
Syrian government forces were on high alert and taking precautionary measures Tuesday at military positions across the country amid fears of a United States strike in the aftermath of the attack near Damascus.
It's armed with Tomahawk cruise missiles, the weapon of choice in a USA attack one year ago on an airfield in Syria following an alleged sarin gas attack on civilians.
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A top Russian diplomat says the Israeli ambassador in Moscow has been "invited" to the Foreign Ministry to discuss developments in Syria following an airstrike that both Moscow and Damascus have blamed on Israel.
The attack comes nearly exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
The watchdog said it has requested the Syrian government to make the necessary arrangements for such a deployment.
The attack in Douma occurred late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group. Last April, after the Syrian government used sarin, a prohibited nerve agent, in an attack on another rebel-held town, Mr. Trump ordered a retaliatory strike against a Syrian air base.
The White House sharply rejected any suggestion that Trump's own words about pulling USA troops out of Syria had opened the door for the attack, which killed more than 40 people, including children.
The Security Council also rejected another Russian-drafted resolution that would have welcomed an investigation by the Organisation for the Prohibition of Chemical Weapons (OPCW) of allegations of a weekend chemical attack in the suburbs of Syria's capital.
Foreign Secretary Boris Johnson said in February that Britain should consider joining military action against Assad's regime if there is fresh "incontrovertible" evidence he has used chemical weapons against his own people.
He also hinted that Syria, Iran and Russian Federation are behind the attack.
"If Russia and other parties, including the (Syrian) regime believe that they were not involved in this attack then they have nothing to fear from the independent investigation the United Nations is seeking", she told ABC radio. Russian military police have since entered parts of the town.