The Organization for the Prohibition of Chemical Weapons, based in the Netherlands, announced it would send a fact-finding team to the site of the attack outside Damascus, and it was due to arrive Saturday.
On the ground in Syria, rebels in Eastern Ghouta surrendered their heavy weapons and their leader left the enclave, signalling the end of one of the bloodiest assaults of the seven-year war and a major win for the Assad regime.
"It's too bad that the world puts us in a position like that", said Trump, as Defense Secretary James Mattis headed to the West Wing to present options for a retaliatory strike.
Trump posted a tweet on Wednesday warning Russian Federation to "get ready" because missiles "will be coming" to Syria.
"We're still assessing the intelligence, ourselves and our allies", Mattis said.
Theresa May received unanimous cabinet backing for United Kingdom military action against Syrian regime targets after senior ministers were briefed by Sir Mark Sedwill, the national security adviser, on the intelligence case pointing to President Assad's culpability for the Douma attack.
"In the past, Trump has condemned others for forecasting military plans, repeatedly blistering President Barack Obama during the 2016 campaign". USA officials have been consulting with France, Britain and other allies on a possible joint military operation, but the timing remained in doubt Wednesday.
The NBC News reports come several hours after French president Emmanuel Macron said he too has "proof" that the attack used chemical weapons.More news: Cleveland hosts Rock Hall Induction Ceremony
His latest tweet, on Thursday evening, said: "Never said when an attack on Syria would take place".
President Donald Trump on Thursday softened his rhetoric about potential airstrikes on Syria, a day after warning Russian Federation that missiles "will be coming, nice and new and 'smart'". There is no reason for this, he wrote, adding that "Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together". Then Thursday he tweeted that an attack on Syria "could be very soon or not so soon at all!"
Trump has slammed Russian Federation for its partnership with "Gas Killing Animal" Assad, spurring concerns that a U.S. strike could lead to a conflagration with Russian Federation, which has major military facilities at Tartus and Khmeimim and works cheek-by-jowl with Syria forces that could be targeted.
"Syria's Foreign Ministry said Wednesday that Trump's threats to attack are "reckless" and endanger global peace and security".
But across Western capitals opposition to military action also grew. The Syrian government has denied the allegations.
But the pinpoint strike did not deter Assad and United States officials, speaking on condition of anonymity, have since investigated as many as 10 suspected chemical attacks.
But British involvement in further military intervention is controversial at home, in a country still haunted by its role in the US-led invasion of Iraq. Multiple IS terror attacks have targeted France, including one last month.