US President Donald Trump waits for the arrival of the Emir of Qatar Sheikh Tamim bin Hamad al-Thani, at the White House in Washington on April 10, 2018. Photo: Associated Press

Responding to Russian warnings to the US against military strikes in Syria, President Donald Trump said Wednesday that missiles "will be coming" in response to Syria's suspected chemical attack that killed at least 40 people.

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

Trump did not detail what a strike would look like, or whether these would be US missiles. The tweet came as Trump administration officials have consulted with global allies on a possible joint military response to Syria's alleged poison gas attack. Trump canceled a foreign trip in order to manage a crisis that is testing his vow to stand up to Syrian President Bashar Assad.

Russian lawmakers have warned the United States that Moscow would view an airstrike on Syria as a war crime, saying it could trigger a direct military clash between the two former Cold War adversaries. Russia's ambassador to Lebanon said any missiles fired at Syria would be shot down and the launching sites targeted - a stark warning of a potential major confrontation in Syria.

The US, France and Britain were in extensive consultations about launching a military strike as early as the end of this week, US officials have said. None of the three countries' leaders had made a firm decision, according to the officials, who were not authorized to discuss military planning publicly.

A joint military operation, possibly with France rather than the US in the lead, could send a message of international unity about enforcing the prohibitions on chemical weapons and counter Syria's political and military support from Russia and Iran.

President Emmanuel Macron said France, the US and Britain will decide how to respond in the coming days. He called for a "strong and joint response" to the attack in the Syrian town of Douma on Saturday, which Syrian activists and rescuers say killed 40 people. The Syrian government denies responsibility.

The French president does not need parliamentary permission to launch a military operation. France is already involved in the US - led coalition created in 2014 to fight the Islamic State group in Syria and Iraq. Multiple IS attacks have targeted French soil, including one last month.

Trump suggested Monday he had little doubt that Syrian government forces were to blame for what he said was a chemical attack, but neither he nor other administration officials have produced hard evidence. Officials suggested such evidence was lacking, or at least not yet at hand. This is in contrast to an incident one year ago in which US intelligence agencies had video and other evidence of certain aspects of the actual attack, which involved the use of Sarin gas. Trump responded by launching Navy cruise missiles at a Syrian airfield.

One official said the US, France and Britain were considering military options that would be more extensive than the punitive, one-day strike last April. That strike did not appear to have had the desired effect of deterring Assad from further use of chemical agents. So the three countries are discussing a range of options, including preventing Assad from conducting future attacks by striking military capabilities involved in carrying out such attack, the official said.

Asked whether France would take military action, Macron said his country will continue discussing technical and strategic information with US and British allies and "in the coming days we will announce our decision." He said any action would "target chemical weapons" stocks. Under a 2013 agreement for which Russia was a guarantor, Syria was to have eliminated all its chemical weapons, but it has used chlorine and perhaps other chemicals since then.

Trump spoke by phone with British Prime Minister Theresa May. A British government statement said the two agreed the attack in Syria was "utterly reprehensible" and that the international community must respond "to uphold the worldwide prohibition on the use of chemical weapons." Trump met at the White House with the emir of Qatar, Sheikh Tamim bin Hamad Al Thani, who told reporters that he and Trump "see eye to eye" on the Syria problem.

"We cannot tolerate with a war criminal," the emir said, adding, "This matter should end immediately." Qatar hosts the United States' main air operations center for the Middle East, which would coordinate any American air attack in Syria.

A watchdog agency, the Organization for the Prohibition of Chemical Weapons, announced that it will send "shortly" a fact-finding mission to Douma, after receiving a request from the Syrian government and its Russian backers to investigate the allegations. It was not immediately clear whether that visit would delay or avert US or allied military action.

The Russian military, which has troops in Syria, said on Monday that its officers had visited the site of the alleged attack and found no evidence to back up reports of poison gas being used.

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! You shouldnt be partners with a Gas Killing Animal who kills his people and enjoys it! Donald J. Trump (@realDonaldTrump) April 11, 2018

Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this. Russia needs us to help with their economy, something that would be very easy to do, and we need all nations to work together. Stop the arms race? Donald J. Trump (@realDonaldTrump) April 11, 2018

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