President Donald Trump appeared to fire a warning shot at Russian Federation on Wednesday morning, saying via Twitter that the country should "get ready" for missiles fired at Syria and that it should not "be partners with a Gas Killing Animal".
The Daily Telegraph newspaper said May had ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military that could begin as early as Thursday night.
Shortly after warning that missiles were coming, Trump tweeted that the USA "relationship with Russian Federation is worse now than it has ever been, and that includes the Cold War".
Trump on Wednesday slammed Russian Federation for its military alliance with Assad, saying it should not "be partners with a Gas Killing Animal who kills his people and enjoys it".
"If it's Russian Federation, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out and we'll know the answers quite soon", he pledged Monday. Under a 2013 agreement for which Russian Federation was a guarantor, Syria was to have eliminated all its chemical weapons, but it has used chlorine and perhaps other chemicals since then.
On Wednesday the Kremlin spokesman Dmitry Peskov urged restraint.
Asked whether the United States military was ready to conduct an attack in Syria if ordered, Mattis replied, "We stand ready to provide military options if they're appropriate, as the president determines".
Mr Trump's decision to announce in highly inflammatory language on Twitter that missile strikes are imminent, rather than retaining an element of surprise, will raise eyebrows among military chiefs.
"The continued use of chemical weapons can not go unchallenged".
Former soldier Johnny Mercer, who sits on the Commons Defence Select Committee, said Britain played a calming role as he criticised the "unhelpful rhetoric" tweeted by the United States president.
One said: 'No money for the NHS, free school meals but there's money for bombing Syria.
The US has been in Syria fighting against ISIS since 2014 in a mission centered around providing air support, equipment, and training for local forces to defeat the terror army.
Asked whether France would take military action, Macron said his country will continue discussing technical and strategic information with USA and British allies and "in the coming days we will announce our decision".
Trump, Emmanuel Macron and Theresa May held phone consultations on Tuesday. But a senior source has told the BBC's North America editor Jon Sopel that any military action is likely to happen at the weekend.
The Syrian government denies responsibility.
British opposition lawmakers are calling for Parliament to be given a vote before any military action.
"If required by our alliance with our partners, we will be there", he said, according to the French translation of his remarks spoken in Arabic.
The rising tensions and the risk of a clash between the major powers were evident in the eastern Mediterranean, where Russian warplanes overflew USA and French naval vessels armed with cruise missiles.