The suspected chemical weapons attack has sparked a fresh diplomatic crisis between Syria, its allies Iran and Russian Federation, and the West over the seven-year Syrian civil war.
It is not the first time Mr. Trump has warned Russian Federation or its president, Vladimir V. Putin, for supporting the Syrian regime, led by Bashar al-Assad, believed to be behind the suspected chemical weapons attack on April 7 that has left dozens dead.
Defense Secretary Jim Mattis, however, indicated that evidence of what happened was still being studied.
Foreign Ministry spokeswoman Maria Zakharova said any United States missile salvo could be an attempt to destroy evidence of the reported gas attack in the Syrian town of Douma, for which Damascus and Moscow have denied any responsibility.
"All the indications are that the Syrian regime was responsible and we will be working with our closest allies on how we can ensure that those who are responsible are held to account". This is in contrast to an incident one year ago in which the USA government had video and other evidence of certain aspects of an actual attack by Syrian aircraft, which involved the use of sarin gas.
Trump on Wednesday even went as far as suggesting that missiles aimed at Syria "will be coming", though no such strike has been publicly announced.
In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 campaign.
In Moscow, the head of a Russian parliamentary defence committee, Vladimir Shamanov, said Russia was in direct contact with the U.S. Joint Chiefs of Staff about the situation. We are totally predictable.
Official said at the time he was moved by images brought to him by his daughter Ivanka and other White House officials.
The U.S. military fired 59 Tomahawk cruise missiles from the Mediterranean Sea, but the strike had little effect as the air base was up and running in just 24 hours.
Last week, Trump advocated pulling USA troops out of the country altogether, a drastic move strongly opposed by top military officials.
Last night however, the White House said the president was still assessing the American response - with all options remaining on the table.
Syria joined Russian Federation in denouncing Mr Trump's threat to attack the country, saying his words were "reckless" and endangered worldwide peace and security.
Relations between Russia and the West have been steadily deteriorating over recent weeks due to Moscow's support for the Syrian government as well as the poisoning of an ex-Russian spy on British soil, which the West has blamed on Russia.
Trump didn't say whether he was referring to a US military strike.
Satellites and other U.S. intelligence aircraft are watching closely for signs that Syrian leader Bashar al-Assad, as well as Russian units - are using the advance notice to move aircraft, weapons and personnel out of the way of a potential attack.
French President Emanuel Macron has called for a "strong, united and resolute" response.
Amid earlier disagreements, a truce collapsed last week and the Syrian government pressed ahead with its offensive. It was not clear whether the presence of the investigators could affect the timing of any USA military action. And he insisted it remains USA policy not to be involved directly in Syria's civil war.
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