Trump, Emmanuel Macron and Theresa May held phone consultations on Tuesday.
"The president and his national security team thought it was best that he stay in the United States while all these developments are taking place", said press secretary Sarah Huckabee Sanders.
"Armed force under mendacious pretext against Syria, where, at the request of the legitimate government of a country, Russian troops have been deployed, could lead to grave repercussions", Mr Nebenzia said. "The U.S. military maintains sufficient countermeasures and protections to ensure the safety of our manned and unmanned aircraft, our forces and the missions they support", said Pentagon spokesman Eric Pahon.
The Syrian government has repeatedly blamed rebel groups for spreading false news in relation to such attacks, and Russian Foreign Minister Sergei Lavrov said allegations of a chemical attack were false and a provocation.
The United States, meanwhile, urged the U.N. Security Council to adopt a resolution that would condemn the continuing use of chemical weapons in Syria "in the strongest terms" and establish a new body to determine responsibility for chemical attacks.
Britain and the United States agreed on Monday that the attack bore the hallmarks of previous chemical weapons attacks by Assad's government, but neither country gave details of what kind of chemical might have been used or how the attack was staged. "Important decisions are being weighed, even as we speak".
Earlier, Trump had condemned what he said had been a "heinous attack on innocent" Syrians, after reports emerged of women and children choking to death in agony, adding: "This is about humanity - it can't be allowed to happen". "Obviously we are working urgently with our allies and partners to assess what has happened on the ground", she said.
Mrs May sidestepped questions about whether Britain would be involved in military action during a visit to Cambridge earlier on Tuesday.
Facebook chief Mark Zuckerberg yet to see 'meaningful impact' from scandal
Facebook Canada said on Wednesday that more than 600,000 Canadians had their data "improperly shared" with Cambridge Analytica . The Sony messages included critical remarks about movie stars and others in the entertainment industry.
On Tuesday the Kremlin's ambassador to the UN, Vassily Nebenzia, repeated his warning to the West against military action in Syria.
The WHO cited reports that 70 people had been killed. It called for an emergency UN Security Council meeting. "We have a lot of options militarily", he told reporters.
The stakes are higher for any new U.S. military action, with Trump explicitly mentioning Iran and Russian Federation in connection with the weekend attack. In a statement, McCain said Trump "responded decisively" a year ago with the air strike and urged Trump to be forceful again to "demonstrate that Assad will pay a price for his war crimes".
In fact, the Assad-Putin-Khamenei axis has a legion of online apologists who argue that the high-profile chemical weapons assaults aren't really the work of the Syrian "president" at all.
Mr Johnson discussed the situation with acting USA secretary of state John Sullivan twice on Monday. The UK and several other states blamed Moscow, and alleged Russian interference in the 2016 U.S. election.
USA officials are assessing whether Syrian President Bashar Assad's government was behind Saturday's suspected chemical weapons attack on rebel-held Douma, east of Damascus. They said the victims had foam on their mouths.
The attack comes nearly exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
Syrian regime and its ally Iran have accused Israel of carrying out the attack on the Tiyas, or T-4, air base near Homs - something that Israel has not confirmed or denied.
That attack prompted US President Donald Trump to order a missile attack on a Syrian air base.