"As the President noted on April 8, the chemical weapons attack by the Syrian regime against innocent civilians in Douma, Syria on April 7 was horrifying, and demands an immediate response from the global community".
Last month, Trump said USA troops would "be coming out of Syria like very soon".
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 he admitted that USA relations with Russia have plunged to a historic low.
James Mattis, US defence secretary, did not rule out any military action against Assad's forces, while US ambassador to the UN, Nikki Haley, warned Washington was ready to "respond" to the attack regardless of whether the Security Council acted or not. Gadi Eisenkot - plan to be in Poland participating in the March of the Living, the annual Holocaust commemoration event at Auschwitz.
Senior officials in the defense establishment warned Iran that Assad would pay the price if Tehran acted against Israel from Syrian territory.
French President Emmanuel Macron said a decision would be made "in the coming days", and that any airstrikes would target the Syrian government's chemical facilities.
Washington and Moscow both have said that a hotline established in 2015 to prevent incidents between their militaries in Syria has worked well, but the rising stakes make the situation more unstable than ever during the Syrian conflict.
Vladimir Chizhov, Russia's permanent representative to the European Union, told Euronews: "Russia has warned USA representatives, both publicly and via corresponding channels, including military ones, about serious consequences that might follow possible strikes (on Syria), if Russian citizens are hurt in such strikes, accidentally or not", Russia's Tass news agency reported.
"The focus right now is definitely on a possible military strike against Syria", said Commerzbank's head of commodity research, Eugen Weinberg. He also condemned Russia's backing of the regime of Syrian President Bashar Assad. It is also unclear what form the operation will take, or how long it will last.
Last year, after another deadly chemical weapons attack on civilians in Syria, Trump authorized a military strike on a Syrian government airbase.
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Russian Federation and Iran deny the claims.
Trump has spoken out against telegraphing United States military plans to enemies in the past.
After Assad's most recent alleged attack, Trump said that "nothing is off the table". In the past, the USA and its Western allies have accused Syria of using sarin gas against civilians, a charge Damascus denies.
While military options are under consideration, a chemical weapons investigative team is on its way to Syria.
"In war, the elememt (sic) of surprise is sooooo important.What the hell is Obama doing", he added later.
It seems the United States and its allies may be preparing for a military strike.
On Wednesday, May said Britain had been working with allies "to get an understanding of what happened on the ground".
There was little expectation that members of Congress would object if Trump launched an attack on Syria, despite some calls for lawmakers to exert their power to authorize military action.
The World Health Organization on Wednesday demanded "immediate" access to the victims of an alleged chemical attack in Syria, voicing indignation at the strike that caused symptoms consistent with exposure to toxic substances.
Brig. -Gen. (res.) Yossi Kuperwasser, of the Jerusalem Center for Public Affairs, told Channel 2 the Americans had no option but to respond with an air strike would be more severe that then one the USA carried out against Syria past year.
Russia's UN envoy VassilyNebenzia, speaking following its veto, said the United States was attempting to mislead the worldwide community. But in the past four weeks alone more than 133,000 people have fled the area, with tens of thousands more still trapped inside its largest town Douma, UN Refugee Agency spokesman Andrej Mahecic said in a statement Tuesday.