In response, Russia's Foreign Ministry said in a Facebook post that "smart missiles should fly towards terrorists, not towards the lawful government".
"Our recommendation to Iran is that it does not try to act, because Israel is determined to pursue this issue to the very end", the officials said.
There were no further details of what the strike would look like, or what kind of missiles will be used.
Trump canceled a trip to Peru and Colombia that was scheduled to start on Friday to oversee the response to the Syria attack, but as of early evening, had made no comment about Syria on Twitter or in his public appearances on Tuesday.
If the United States does intend to strike targets under Russia's air defenses, it will carry out perhaps the most complicated, technologically advanced military skirmish of all time. It is also unclear what form the operation will take, or how long it will last.
"We stand ready to provide military options if they are appropriate as the President determined", he said.
Asked whether the USA military was ready to conduct an attack in Syria if ordered, Mattis replied, "We stand ready to provide military options if they're appropriate, as the president determines".
He called for a "strong and joint response" to the attack in Douma on Saturday, which Syrian activists and rescuers said killed 40 people.
Russian Federation and Iran deny the claims.
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Donald Trump cancels trip to Summit of Americas in Peru
Instead, Vice President Mike Pence will lead the US delegation to the Summit of the Americas that kicks off Friday in Lima, Peru. Last year, Washington bombed a Syrian government air base after a chemical weapons attack.
"The Russian military in Syria has air defence systems theoretically capable of shooting down US Tomahawk missiles but these can be saturated and in the case of the S-400 [another Russian air defense system] in particular, are largely unproven in actual combat use", Justin Bronk, an air combat expert at RUSI, told Business Insider.
His failure to secure it prompted then-US President Barack Obama to request congressional approval for a US strike against Assad.
Speaking in the British city of Birmingham on Wednesday, May said "all the indications" are that the Syrian regime was responsible for a chemical weapons attack in Douma, adding that the use of such weapons "cannot go unchallenged".
"What he's actually saying is, if the U.S. and allies decide to strike against Syrian chemical weapons and delivery aircraft, not only are they going to try and shoot down the missiles in flight".
The Prime Minister did not indicate whether Britain will be involved in the military campaign but insisted continued attacks "cannot go unchallenged" as she pledged to continue working with the UK's "closest allies" on how the dictatorship is held to account.
Fellow Tory MP John Baron said any "substantial military commitment" should be approved by Parliament before any action.
On Wednesday, the UN's World Health Organization demanded access to verify reports from its partners, which include Sams, that 70 people had died - including 43 who showed "symptoms consistent with exposure to highly toxic chemicals".
In Kremlin, everyone supports an objective investigation of the incident in Syria's Douma, he said.
It seems the U.S. and its allies may be preparing for a military strike.