The decision by the United States to strike marked Mr Trump's second order to attack Syria; he authorised a barrage of Tomahawk cruise missiles to hit a single Syrian airfield in April 2017 in retaliation for Syria's use of sarin gas against civilians. "Can not allow it", Trump told reporters during a Cabinet meeting on Monday, adding he expected to make a decision on a response in the next 24 to 48 hours.
Syrian troops have braced for Western strikes across the country, hiding assets and deserting key buildings.
Earlier in the day, Bashar al-Jaafari, Syrian ambassador to the U.S., said in NY that two investigating teams from the OPCW were scheduled to arrive in Syria within the next 24 hours.
Israel said the attack was "justified", while Iran said it would have regional consequences.
The White House says it is continuing to assess intelligence and talk to its allies on how to respond.
Why is the West considering military action?
Meanwhile, leading officials from the Organisation for the Prohibition of Chemical Weapons have requested access to the site of the alleged chemical weapons attack on a rebel-held neighborhood in Syria on Saturday.
US officials said there was no indication so far that Syrian air defenses struck attacking Western aircraft or ships.
In an early response, the Russian foreign ministry called the Western attacks a violation of global laws.
On Thursday, US officials were quoted as saying that samples from victims had tested positive for chlorine and a nerve agent.
The decision to strike, after days of deliberations, marked Trump's second order to attack Syria; he authorized a barrage of Tomahawk cruise missiles to hit a single Syrian airfield in April 2017 in retaliation for Assad's use of sarin gas against civilians.
The attack has drawn worldwide outrage and prompted the USA and its allies to consider a military strike on Syria, something Moscow has strongly warned against. It is not about regime change.
Trump called the incident a "heinous attack on innocent" Syrians and vowed that the U.S. would respond: "This is about humanity; it can't be allowed to happen".
Rasmussen said he was pleased to see the US, UK and France were working closely together to come up with a plan of action and that Denmark would offer its clear support to a response.
In an April 11 tweet, Trump told Russian Federation to "get ready" because missiles "will be coming". But Thursday, he seemed to walk back that statement, tweeting military action "could be very soon or not so soon at all".
White House press secretary Sarah Huckabee Sanders issued a statement late Thursday, following the meeting with Mattis and national security advisers. "We will try to make it better, but it is a troubled place". "They'll be made fairly soon".
Russian President Vladimir Putin listens during a meeting with Health Minister Veronika Skvortsova and Alexander Bastrykin, the head of Russia's Investigative Committee, in Moscow on March 28, 2018.
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