Russian Federation is a major backer of the government of Syrian President Bashar al-Assad in the country's civil war, and strongly opposes the US claim that Syrian government forces attacked a rebel-held town near Damascus on Saturday with chemical weapons.
Should the President follow through on his warnings of an attack, two US Navy destroyers armed with Tomahawk cruise missiles are in position and ready to be called into action, among other assets including jets and submarines.
According to media reports, a Russian official said that Russia is ready to shoot down any and all missiles fired at Syria by the United States, while the US side threatened military options at any time.
White House press secretary Sarah Huckabee Sanders insisted no final decisions had been taken and that "all options are on the table".
But a special hotline for the U.S. and Russian militaries to communicate about operations in Syria is active and being used by both sides, Moscow said Thursday.
Syria and Russia have denied using poisonous gas in Douma on April 7, with Russian Foreign Minister Sergei Lavrov saying that Moscow had evidence that the attack in Douma was staged.
The attack was first reported by Syrian rebel group Jaish al-Islam on Saturday.
At the United Nations meanwhile, diplomats were mulling a draft resolution put forward by Sweden and obtained by AFP, that would dispatch a "high-level disarmament mission" to rid the country of chemical weapons "once and for all".
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. "All the indications are that the Syrian regime was responsible".
Speaking on TF1 television, Macron said that France would not tolerate "regimes that think everything is permitted".
British Prime Minister Theresa May's senior ministers agreed on the need for action at a Cabinet meeting on Thursday, but Downing Street did not specify what measures the United Kingdom would take, reports CNN.
It is a military decision and one with enormous risk that could trigger a Russian retaliation, ITV's Robert Moore reports. USA lawmakers questioned whether Trump has the legal authority to order strikes without Congressional approval and opposition parties voiced concern.
In April previous year Trump ordered Tomahawk strikes on the Shayrat Airbase in response to a similar chemical weapons attack on rebel-held Khan Sheikhun.
Echoing the U.S. stance, France said Assad's government had reached a "point of no return" with repeated use of chemical weapons.
The same officials say Syria has continued to produce or procure chlorine, which also has industrial and agricultural uses.
Citing US estimates that Assad has used chemical weapons "at least 50 times" in the seven-year war, Haley said: "All nations and all people will be harmed if we allow Assad to normalize the use of chemical weapons". "People started shouting in the streets, 'Chemicals!"