Trump met with members of his National Security Council on Thursday to discuss the US's options with regards to Syria.
Mattis, who was spotted alongside CIA Director Mike Pompeo at the White House on Wednesday, said he stands "ready to provide military options if appropriate".
"We don't have any open source information which can verify Syrian movement [of military equipment], but we are supposing this is true", said Ruslan Leviev, a Moscow-based military researcher with Conflict Intelligence Team, a research organisation which investigates conflicts in Syria and Ukraine.
"What we've got here in Syria is a choice between monsters on the one hand and maniacs on the other", Julian Lewis, the chairman of the House of Commons defence committee, told the BBC.
The Russian military, which has troops in Syria, said on Monday that its officers had visited the site of the alleged attack and found no evidence to back up reports of poison gas being used.
Later Wednesday morning, Mr. Trump tweeted that Washington's relationship with Moscow is "worse now than it has ever been".
The ongoing deliberations reflected the conundrum Trump faces as he contemplates wading further into Syria's multi-sided civil war.
At stake in Syria is the potential for confrontation, if not outright conflict, between the U.S. and Russian Federation, former Cold War foes whose relations have deteriorated in recent years over Moscow's intervention in Ukraine, its interference in the 2016 USA presidential election and its support for Syrian President Bashar Assad.
May's office said in a statement that the Cabinet judged it "highly likely" the government of Syrian President Bashar Assad was behind the chemical attack last weekend in the town of Douma.
At a UN Security Council meeting US Ambassador Nikki Haley warns Washington is determined to "see that the monster who dropped chemical weapons on the Syrian people is held to account".
British minister Davis said his decision then to vote against action was based on a lack of clear evidence and a lack of a clear plan.
"We are very concerned with the situation, with the risky escalation, with the we hope that there will be no point of no return. Now we have to make some further decisions". "So they'll be made fairly soon".
The Trump administration, he said, has not yet made any decision to launch military attacks into Syria. On Wednesday, Trump appeared to suggest airstrikes were imminent, warning Russia, Syria's key ally, to "get ready" for military action. It is very hard not to see Trump move in a direction where he pushes the U.S.to withdraw from the nuclear agreement.
Foreign Minister Jean-Yves Le Drian said Thursday that President Emmanuel Macron would decide whether to launch an attack over the "non-respect of the global convention against chemical weapons", which is a "red line" for France.
The statement said "more than 500 cases" showed "symptoms indicative of exposure to a chemical agent". Officials said Thursday the trio has yet to reach firm agreement on the scale or timing of a response.
Asked if he meant war between the United States and Russian Federation, he said: "We can not exclude any possibilities unfortunately".
A French frigate, UK Royal Navy submarines laden with cruise missiles and the USS Donald Cook, an American destroyer equipped with Tomahawk land attack missiles have all moved into range of Syria's sun-bleached coast.
A Kremlin spokesman has said claims of a chemical weapons attack were fabricated. Macron specified that "at least chlorine" was used by the Assad regime.
Russian Federation has warned the West against attacking its Syrian ally President Bashar al-Assad, who is also supported by Iran, and says there is no evidence of a chemical attack in the Syrian town of Douma near Damascus.
Those comments go further than U.S. officials have been willing to in public.
Putin's Russian Federation has stood in the way of all attempts to hold Assad accountable for his crimes, including just two days ago when it blocked a United Nations resolution that would have created an independent investigation into chemical weapons usage in Syria.
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