In response to Mr Trump's tweet, the Pentagon said on Wednesday that it would not comment on potential future military operations.

"The department does not comment on potential future military actions. I refer you to the White House to characterise the President's tweet," Eric Pahon, a Pentagon spokesman, said in a statement.

"As the President noted on April 8, the chemical weapons attack by the Syrian regime against innocent civilians in Douma, Syria on April 7 was horrifying, and demands an immediate response from the international community."

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. Mr Trump cancelled a foreign trip to manage a crisis that is testing his vow to stand up to Syrian President Bashar al-Assad.

Russian lawmakers have warned the US 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 authorised 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.

French President Emmanuel Macron said France, the US and Britain would decide how to respond in the coming days. He called for a "strong and joint response" to the attack in the town of Douma on Saturday. The Syrian government denies responsibility.

Mr Macron 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.

Mr Trump suggested on Monday that 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. Mr 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.

Asked whether France would take military action, Mr Macron said his country would 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.

Mr Trump spoke by phone with British Prime Minister Theresa May. A British government statement said the two agreed that 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".

The President also met at the White House with the emir of Qatar, Sheikh Tamim bin Hamad Al Thani, who told reporters that he and Mr Trump "see eye to eye" on the Syria problem.

"We cannot tolerate a war criminal," the emir said, adding, "This matter should end immediately." Qatar hosts the United States' main air operations centre 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 would "shortly" send 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.

AP, Reuters