The Syrian government called his threats to attack "reckless", claiming they endanger worldwide peace and security, a report broadcast on state TV said.
The UN Security Council has scheduled another emergency meeting, scheduled for Friday at 10am NY time (midnight Saturday AEST), at Russia's request.
"Get ready Russian Federation", the president said.
"We're still assessing the intelligence, ourselves and our allies", Mattis said.
France warned Friday (April 13) that Syria's government had reached a "point of no return" by using chemical weapons and vowed to confront the "intolerable threat" of such attacks to global security.
"The department does not comment on potential future military actions".
Trump wrote in an incendiary early morning tweet that the missiles would be "nice and new and smart!", after a Russian official said any strike in retaliation to the attack on the rebel-held town of Douma would be shot down.
During the two-hour meeting yesterday afternoon, ministers agreed to join forces with the U.S. and France to prevent Syrian leader Bashar al-Assad's forces from launching an "appalling and inhumane" attack again.
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But the Violations Documentation Center (VDC), which records alleged violations of worldwide law in Syria, said bodies were found with oral foaming, cyanosis, and corneal burns.
Moscow and the Syrian regime deny that the gas attack took place or that either regime was involved.
During a phone call late on Thursday, UK Prime Minister Theresa May and US President Donald Trump agreed on the need to deter chemical weapon use in Syria.
What has Trump said about the attack?
Trump's latest threat is a response to comments made by Alexander Zasypkin, Russia's ambassador to Lebanon, who on Tuesday evening said Russia would respond to any USA strike on Syri by targeting any missiles and launchers.
Mr Trump, who has cancelled a planned trip overseas, has been canvassing support for strikes from the leaders of France and the UK. It "could be very soon or not so soon at all", he said.
Speaking to congress at a hearing of the House Armed Services Committee on Thursday, Mattis said he believed a chemical attack had taken place and blamed Russian Federation for being complicit in Syrian government's possession of chemical weapons. "The president has not made that decision". "Final decisions haven't been made yet on that front", Sarah Sanders said, adding that the President has not laid out a timetable.
The possibility of a military strike on Syria was discussed by the United States and its partners, particularly France and the United Kingdom, in the wake of reports on an alleged chemical weapons attack in the Syrian city of Duma.
The US president, whose military would nearly certainly provide most of the intelligence and weaponry, has called for withdrawing US forces globally but also for hitting enemies hard.