Satellites and other USA intelligence aircraft are watching closely for signs that Syrian leader Bashar al-Assad, as well as Russian units - are using the advance notice to move aircraft, weapons and personnel out of the way of a potential attack.
Macron has said that any French strikes against the Syrian regime would target chemical facilities, and a British government official told CNN on Wednesday that preparations are being made by the United Kingdom for possible action but cautioned the political decision has not yet been made. Any missile attack that puts Russian lives at risk, Moscow has said, would result in Russia striking back at the missiles and at the planes or ships that launched them.
The Russian military announced on Thursday that the Syrian government is now in full control of a town on the outskirts of Damascus that was held by the rebels and that was the site of suspected chemical attack over the weekend.
Trump slammed Russian Federation on April 11 for its military alliance with Assad, saying it should not "be partners with a Gas Killing Animal who kills his people and enjoys it".
Brexit Secretary David Davis, speaking in London, said no decision had been taken on whether there would be a joint response by the United Kingdom and USA and other allies.
The Prime Minister faces backlash amid signs she is preparing to join US-led air strikes against the Assad regime in Syria, as people tweet #NotInMyNameTheresaMay. The OPCW replied that a mission would be deployed to the affected town and asked the Syrian government to make necessary preparations. Trump responded then by launching dozens of Navy cruise missiles at a Syrian airfield.
"We stand ready to provide military options if they are appropriate as the President determined", he said.
Mattis was to meet with Trump later on Wednesday.
But in tweeting about a potential attack, Trump appeared to publicly telegraph military plans - something for which he heavily criticized former President Barack Obama back in 2013.
"Potentially we could be past the worst here", Mark Nash, head of fixed income at Old Mutual Global Investors, says in interview on Bloomberg TV. We are totally predictable.
The Russian military said on Wednesday that it was closely monitoring the situation around Syria and was aware of the movements of a USA naval force in the area.
Meanwhile, Syrians are bracing for a possible US attack in retaliation to an alleged chemical assault that killed dozens on Saturday in Douma.
U.S. Secretary of Defense Jim Mattis was at the White House Wednesday as Vice President Mike Pence convened the president's national security team for a meeting to contemplate the latest use of force against Syrian President Bashar al-Assad. "We're still working on this".
In response, the Russian Foreign Ministry said that missiles "should fly toward terrorists, not toward the lawful government" of Syria.
Trump's administration has sought to show toughness on Russian Federation, with a series of economic and diplomatic actions, including new sanctions last week against government officials and oligarchs.
Moscow's threat to down U.S. missiles came from its ambassador to Lebanon, Alexander Zasypkin, who said it was based on previous statements by President Vladimir Putin and the Russian armed forces chief of staff.
US officials have refused to rule out direct military engagement with Russian Federation, with the White House saying that "all options are on the table".
Syria, Iran and Russian Federation say Israel was behind an air strike on a Syrian air base on Monday that killed seven Iranian military personnel, something Israel has neither confirmed nor denied. There was also no indication that Syrian government forces had entered Douma on Thursday, the Associated Press reported.
But any further military role would require parliamentary approval.