Trump spoke with other world leaders, and other U.S. officials said the U.S., France and Britain were in extensive consultations about launching a military strike as early as the end of this week.
The investigators, who are only mandated to determine if chemical weapons were used and not who used them, were expected to start their investigations into the Douma incident on Saturday, the Netherlands-based organisation said.
Trump threatened to take action within 48 hours, and speculated that Russian Federation or Iran could have been worked with Syrian president Bashar al-Assad to launch the killer gas.
President Donald Trump has threatened military action, vowing to respond "forcefully" to Saturday's attack on civilians in the town of Douma and warning that Russian Federation - or any other nation found to share responsibility - will "pay a price".
Russian lawmakers have warned the United States that Moscow would view an airstrike on Syria as a war crime and that it could trigger a direct U.S-Russian military clash.
The presence of Russian troops and air defenses in Syria were among numerous complications weighing on Trump, who must also consider the dangers to roughly 2,000 American troops in the country if Russia were to retaliate for USA strikes.
Mattis said he believed chemical weapons had been used, but "we are looking for evidence" on who was responsible.
Russian state news agencies are reporting that a high-level Russian delegation has arrived in Syria where it is going to meet with President Bashar Assad.
"The Failing New York Timeswrote another phony story".
The UN Security Council, tasked with maintaining worldwide peace and security, has been riven, with Moscow virulently denying the Douma attack took place, or postulating that it was carried out by rebels.
The talks stressed that threats of some Western countries to attack Syria, "based on the lies fabricated by these countries and their tools of the terrorist organizations inside Syria, came after the liberation of the eastern Ghouta and the failure of a new bet of those upon which these countries have relied in the war on Syria". The resolution was likely to be vetoed by Syrian ally Russian Federation.
State news agency RIA Novosti on Wednesday quoted Andrei Krasov, deputy chairman of the State Duma's defense committee, as saying that Russian Federation will treat a USA airstrike on Syria "not just as an act of aggression but a war crime of the Western coalition". The danger of hitting Russian forces in Syria is also real - Russian military advisors are embedded in many areas with Syrian forces and located at government facilities.
Trump appeared to be taunting Russian Federation with American firepower he believes is superior.
That did not necessarily signal, however, that Trump was cooling to the idea of military action, especially given the high stakes in Syria.
"We are doing things that nobody thought possible, despite the never ending and corrupt Russia Investigation, which takes tremendous time and focus".
Syria's military has repositioned some air assets to avoid fallout from possible missile strikes, USA officials told Reuters on Wednesday.
Was this tweet a purposeful kick-off of a missile launch?