In Moscow, the head of a Russian parliamentary defense committee, Vladimir Shamanov, said Russia was in direct contact with the US Joint Chiefs of Staff about the situation.
One woman who refused to be identified because of fear for her security described herself as "trembling" and said she stocked up on food and is considering not sending her kids to school.
Douma and the sprawling eastern Ghouta region near the capital, Damascus, had been under rebel control since 2012 and was a thorn in the side of President Bashar Assad's government, threatening his seat of power with missiles and potential advances for years.
The Cook - named after a Marine Colonel who suffered depravation and starvation as a Vietnam prisoner of war - has past experience tangling with the Russian military, having been deployed to the Black Sea during the recent crisis in Crimea.
Trump's spokeswoman dismissed this idea, and pointedly refused to acknowledge that concern about the risks of a direct confrontation with Russian Federation would hold the USA military back.
In a second tweet, Trump said: "Our relationship with Russian Federation is worse now than it has ever been, and that includes the Cold War".
Trump's tweets were more belligerent - he told Russia "You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" - and declared that USA relations with Russia have plunged to a historic low.
White House press secretary Sarah Sanders insisted on Wednesday that a missile strike remains one of the many options on the table and said the President's tweet does not complicate the planning of a USA response.
After the Douma attack, the insurgent group dug in there, Jaish al-Islam, finally agreed to withdraw.
At the United Nations meanwhile, diplomats were mulling a draft resolution put forward by Sweden and obtained by AFP, that would dispatch a "high-level disarmament mission" to rid the country of chemical weapons "once and for all".
The Foreign Ministry in Damascus denounced Trump's threat as "reckless" and a danger to global peace and security. But concrete evidence linking the Syrian regime to the attack might not be a prerequisite for a military response from the West.
The French president has consistently said he'd act, alone if necessary, if Mr Assad crossed the "red line" of using chemical weapons again.
British ministers were also due to discuss whether to join the United States and France in military action on Thursday, while Russian military officials said Douma was now fully in the hands of government forces.
Post-World War II Germany typically has been reluctant to engage in military action, and parliamentary approval is required for any military missions overseas. "Even if the U.S. delivers a strike, this won't change the power balance in Syria", the MP said.
Syria and its main ally, Russia, have denied that any chemical weapons were used and gave permission for the OPCW to investigate.
The President has been mulling strikes against the Assad regime for suspected use of chlorine-based chemical weapons against civilians Saturday.
Trump condemned the chemical attack in Syria and said that whoever is responsible for the attacks must be held accountable for these human rights abuses.
The World Health Organization said about 500 patients showed signs of exposure to toxic chemicals following the Douma attack.