WASHINGTON — President Donald Trump signaled Wednesday that he was on the verge of ordering a barrage of “nice and new and ‘smart!’ “ missiles against the Syrian military for its suspected use of poison gas against civilians, dismissing a threat that Russian forces would shoot down U.S. missiles in flight.

The warning came as U.S. and French warships armed with cruise missiles are in the eastern Mediterranean off the Syrian coast, and activists said that Syrian forces had begun evacuating air bases and other military facilities in anticipation of a U.S.-led air raid aimed at punishing the government in Damascus and crippling its chemical weapons capabilities.

Defense Secretary James N. Mattis and other national security aides huddled at the White House as leaders in Britain, France and Saudi Arabia pledged support for a strong response to the suspected poison gas attack Saturday that killed more than 40 people, including children, in a rebel-held town outside Damascus.

Mattis suggested that Trump had not yet decided to launch the Tomahawks. “We’re assessing the intelligence _ ourselves and our allies,” he told reporters. “We’re ready to provide military options if they are appropriate, as the president determines.”

Any air assault is expected to be more extensive and more damaging than Trump’s decision last April to fire 59 Tomahawk cruise missiles against a single Syrian airfield in response to a nerve gas attack that month. The airfield was back in operation soon after.

The crisis in Syria has sparked a new level of tension between Washington and Moscow, raising concern that the dispute over a suspected chemical attack could spiral into a deadly conflict between the former Cold War enemies. Both countries have several thousand troops deployed in Syria.

Trump fanned those fears early Wednesday with tweets aimed at the Kremlin and Syrian President Bashar Assad, whom Russia supports in the country’s 7-year-old civil war.

“Russia vows to shoot down any and all missiles fired at Syria. Get ready Russia, because they will be coming, nice and new and ‘smart!’ “ Trump tweeted. “You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!”

Trump tweeted after a mid-level Russian diplomat defiantly claimed that Russian forces in Syria would shoot down U.S. missiles in midair, and suggested that Russian President Vladimir Putin had issued the order. The Kremlin did not confirm that Putin had done so.

“Russia will execute the statement of its president related to any U.S. aggression against Syria, knocking down American missiles and striking at the sources of fire,” Alexander Zasypkin, Russia’s ambassador to Lebanon, told the Lebanese television channel Al Manar.

Trump’s tweet was striking given his reluctance over the last year to criticize Putin. He has largely remained silent even as his administration has imposed sanctions on more than 100 Russians, including some in Putin’s inner circle, and expelled scores of accused Russian spies, in retaliation for Russian meddling in the 2016 election, its cyberattacks and other malicious activities.

Though none of his tweets Wednesday mentioned Putin by name, Trump declared that “our relationship with Russia is worse now than it has ever been, and that includes the Cold War.”

Citing U.S. intelligence, White House Press Secretary Sarah Huckabee Sanders said Trump “holds Syria and Russia responsible for this chemical weapons attack.”

Asked for evidence of Russia’s role, she said Moscow “holds some responsibility” because it guaranteed that Syria wouldn’t use chemical weapons again as part of a 2013 agreement. She did not say whether Russia had played a direct role in Saturday’s attack, which reportedly involved Syrian helicopters dropping barrel bombs on the rebel-held town of Duma.

She declined to label Russia an “enemy” when asked, choosing instead to say “we certainly think they’ve proven to be a bad actor.”

In a later tweet, Trump blamed “much of the bad blood with Russia” on “the Fake & Corrupt Russia Investigation, headed up by the all Democrat loyalists, or people that worked for Obama.”

Trump’s mixing of a tense global crisis with his domestic frustrations marked another unprecedented moment in a presidency that seems headed for its largest military reprisal to date.

A national security analyst said Trump’s bellicose tweets could be a complicating factor for the Pentagon because commanders must ensure “the size of the strike matches the rhetoric.”

The analyst, who spoke anonymously to avoid burning bridges at the White House, said he believed Trump dropped his public deference to Putin after the Russian leader forged a nominal alliance in Syria with Turkey and Iran, leaving the United States on the sidelines as the civil war moves into its final stages.

“There was an opportunity for a major power, almost summit-style Syria endgame, and I think that’s what Trump was playing for,” the analyst said.

As tensions escalated, the Kremlin said it would not participate in “Twitter diplomacy.”

“We are supporting serious approaches,” said Putin’s spokesman Dmitry Peskov, Russian news agencies reported. He said claims that chemical weapons were used “are far-fetched and cannot excuse any use of force” in response.

Peskov said Russian officials want an “unbiased” investigation into accusations of a chemical attack, “so that we do not take guidance from rumors and empty reports in the media.”

Russian lawmakers on the parliament’s defense committee said Moscow would have an “immediate response” to any U.S. attack on its military in Syria. Russia has ground-based air defense systems in Syria and a naval base on the eastern Mediterranean.

Trump planned to meet late Wednesday with Republican congressional leaders, who have shown little desire to weigh in on military strikes or to get more deeply involved in Syria. Only a handful of lawmakers have demanded a vote to authorize military action.

“Before President Trump conducts military operations, he must come to Congress for authorization,” Sen. Edward J. Markey, D-Mass., said in a statement that called Trump’s tweet “the height of irresponsibility.”

In August 2013, President Barack Obama sought authorization from a deeply skeptical Congress to launch airstrikes against Syria to punish it for a poison gas attack after he failed to win support from Britain, a usually reliable international partner. Congressional leaders refused to call a vote, however, and the attack was canceled.

Trump’s tweets suggesting an imminent attack ran counter to his public boasts that he does not telegraph military operations because it ruins the element of surprise.

Military experts say the strategic value of a surprise attack is often outweighed in modern warfare by the need to protect civilians and allied groups that may be near opposition targets.

U.S. officials are still trying to determine whether Syria used a banned nerve agent, such as sarin, in the attack on Duma, as well as a toxic gas, such as chlorine, which is deadly but not barred by international agreement.

Local medics and rescue workers said some of the victims smelled of chlorine. Others showed symptoms of exposure to an organophosphate, including constriction of the pupils and convulsions, leading to speculation that sarin was used as well.

“Syria is not supposed to have sarin at all,” said Andrew Tabler, a Syria expert at the Washington Institute for Near East Policy, a nonpartisan think tank. “And if they find it, the U.S. may feel compelled” to carry out more punishing attacks.

Syrian government troops evacuated airports and primary military air bases in government-held areas across the country, according to the Syrian Observatory for Human Rights, a pro-opposition group based in England that relies on activists in Syria.

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The Syrian government also warned paramilitary groups aligned with the Syrian army _ including troops from Iran, Lebanon, Iraq and Afghanistan _ to leave their bases, the Syrian Observatory group said.

Russia reportedly sent troops into Duma, where the suspected gas attack took place. The Damascus suburb was the last area to succumb to a Russian-backed offensive aimed at smashing the rebels’ grip on east Ghouta.

A planned evacuation from the town was postponed. Qais Hassan, an opposition activist in Duma, said no buses entered for several days to evacuate thousands of people intent on leaving the former rebel enclave.

Writing on social media, Syrians reported an uptick in warplanes in Syria’s skies, while others tweeted about civilian flights being diverted in advance of a U.S. assault.

On the streets of Damascus, a number of residents insisted it was business as usual, however.

“It’s the usual traffic jam here ... and there have been no visible movements by the military,” Nicholas Zahr, a Damascus resident, wrote on Facebook.