The U.S., France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, U.S. officials have said. "Could be very soon or not so soon at all!"
In Paris, France's Emmanuel Macron upped the pressure on Moscow by stating he had "proof" that the Assad's regime had used chemical weapons, and vowing a response "at a time of our choosing".
Trump is considering his military options in Syria after Saturday's alleged chemical attack against the rebel-held town of Douma.
It comes after a chemical weapons attack on the rebel-held city of Douma at the weekend, which killed some 70 people and injured up to 500 more.
But across Western capitals opposition to military action also grew.
"It is Congress, not the President, who determines whether our country goes to war and Congress must not abdicate that responsibility".
The French president does not need parliamentary permission to launch a military operation.
In the call, the two leaders had agreed that the regime of Syrian President Bashar al-Assad "had established a pattern of risky behaviour in relation to the use of chemical weapons", Downing Street said.
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"Now we have to make some further decisions, so they will be made fairly soon", added first-term commander-in-chief, who earlier appeared to equivocate on the timing of strikes.
Moisi stressed the "risk of escalation" of the conflict amid increasing concerns about a U.S. -Russia proxy war. He continued by saying that worldwide law has been systematically violated in Syria and that he feels angry at reports of chemical weapons use in the country.
The Secretary-General of the United Nations Antonio Guterres said at the session that the worsening situation in the Middle East is threatening global peace and security.
After meeting for more than two hours, the Cabinet gave May the green light to join the USA and France in planning possible strikes, but also left open the possibility of other responses.
Opposition lawmakers have called on May to give Parliament a vote before committing British troops.
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
Post-World War II Germany typically has been reluctant to engage in military action, and parliamentary approval is required for any military missions overseas.
But they backed action in Iraq the following year, and again in Syria in 2015, strictly limiting strikes to Islamic State (IS) group targets.
"I think it is important to have a common line, without Germany participating militarily", Merkel said.
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. France has 650 troops based in the United Arab Emirates overall.
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.