Worries about a confrontation between Russia, Syria's big ally, and the West have been running high since Trump said that missiles "will be coming" in response to the suspected chemical attack in the Syrian town of Douma on April 7.
President Donald Trump put off a final decision on strikes against Syria following a crunch meeting with national security advisors Thursday, as Moscow warned against any US move that could trigger a conflict between the nuclear-armed rivals.
Minutes after Pentagon brass and cabinet members jumped in black armored SUVs and sped away from the White House, Press Secretary Sarah Sanders said Trump would continue to assess intelligence and engage with allies.
"No final decision has been made," she said, adding that Trump was scheduled to confer with French President Emmanuel Macron and British Prime Minister Theresa May.
Later, a Downing Street spokesperson confirmed that Trump and May had agreed during a phone call to "keep working closely together on the international response" over Syria.
"They agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime," the spokesperson added.
TRT World's Reagan De Vignes reports.
The military drumbeat appeared to reach crescendo ahead of the meeting, with US action seeming imminent as Russia stonewalled diplomatic efforts at the United Nations and France cited "proof" that Moscow's Syrian ally carried out a deadly chemical weapons attack.
Western officials believe chlorine was used in a Saturday attack on Douma, the main city in the longtime rebel bastion of Eastern Ghouta, where the British government now estimates 75 people were killed, and that Bashar Al Assad's regime was responsible.
What is less clear is whether sarin or a sarin-like agent was also used.
Never said when an attack on Syria would take place. Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our “Thank you America?”— Donald J. Trump (@realDonaldTrump) April 12, 2018
During the West Wing sitdown, Defense Secretary James Mattis and the Chairman of the Joint Chiefs of Staff Joe Dunford were expected to lay out options for retaliatory strikes against the Syrian regime.
But after a week in which Trump has burst through an almost daily series of self-imposed decision deadlines, he again appeared unwilling to rush headlong to war.
Just hours earlier Trump had vowed to make his choice "fairly soon."
His window for military action could be narrowing, with inspectors from the Organisation for the Prohibition of Chemical Weapons expected to arrive in Syria to begin work on Saturday, following an invitation from Damascus.
Diplomats have expressed concern that those experts could be used as hostages or human shields.
Since Saturday, when images of ashen toddlers struggling for breath emerged there has been a sustained military buildup in the eastern Mediterranean.
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.
In New York, Russia's UN ambassador warned the priority in Syria was to avert US-led strikes that could lead to a confrontation between the world's two preeminent nuclear powers.
"The immediate priority is to avert the danger of war," said Ambassador Vassily Nebenzia following closed-door Security Council talks, refusing to exclude the possibility of US-Russian clashes.
Trump has slammed Russia for its partnership with "Gas Killing Animal" Assad, spurring concerns that a US strike could lead to a conflagration with Russia, which has major military facilities at Tartus and Khmeimim.
US officials have refused to rule out direct military engagement with Russia, with the White House saying "all options are on the table."