Donald Trump has hit out at Russia after Moscow threatened to shoot down any missiles fired at Syria in response to an alleged chemical weapons attack.

On Twitter, the US president told Russia to "get ready", saying: "They will be coming, nice and new and 'smart'!"

Referring to Syrian president Bashar al-Assad, he added: "You shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it!"

Though he failed to detail what a strike would look like, or whether they would be US missiles, Mr Trump's apparent confirmation that military action will be taken came after Russian lawmakers warned Moscow would view American air strikes on Syria as a war crime.

Mr Trump's outburst appeared to be a response to Russia's ambassador to Lebanon, Alexander Zasypkin, who said US launch sites as well as missiles would be targeted in the event of an attack.

"If there is a strike by the Americans then... the missiles will be downed and even the sources from which the missiles were fired," Mr Zasypkin told Hezbollah's al-Manar TV on Tuesday evening.

The US president has previously threatened military action in response to Syria's suspected poison gas attack, which activists and rescuers say killed at least 40 people.

The Syrian government and its Russian backer deny the attack took place.

General Richard Barons on Russia's claim it will shoot down US missiles fired at Syria: 'That is war'

Responding to Mr Trump's tweet, Russian foreign ministry spokeswoman Maria Zakharova said on Facebook: "Smart missiles should fly towards terrorists, and not towards the lawful government which has been fighting international terrorism on its territory for several years."

She added a US missile strike could be an attempt to destroy evidence of the alleged chemical weapons attack in the Syrian town of Douma, near Damascus.

Mr Trump later sent a follow up tweet in which he described the US-Russia relationship as "worse now than it has ever been".

"There is no reason for this," he wrote. "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?"

The tweets came as Trump administration officials consult with global allies over a possible joint military response to Syria's alleged poison gas attack.

The White House announced on Tuesday Mr Trump would skip an upcoming summit in South America and instead remain in the US to "oversee the American response to Syria”.

The US, France and Britain have been in extensive consultations about launching a military strike as early as the end of this week, US officials 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.

Eastern Ghouta bombings: Syrian war in pictures Show all 14 1 /14 Eastern Ghouta bombings: Syrian war in pictures Eastern Ghouta bombings: Syrian war in pictures A Syrian woman and children run for cover amid the rubble of buildings. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Smoke rises from buildings following the attack on the village of Mesraba in the rebel-held besieged Eastern Ghouta region on the outskirts of the capital Damascu. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Injured children receive medical treatment. EPA Eastern Ghouta bombings: Syrian war in pictures A Syrian man carries a child injured. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures An injured child receives treatment following bombings on several areas of eastern Ghouta. EPA Eastern Ghouta bombings: Syrian war in pictures A child reacts inside a hospital after relatives were injured in the bombing. EPA Eastern Ghouta bombings: Syrian war in pictures Syrian children cry at a make-shift hospital in Douma following air strikes on the Syrian village of Mesraba. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Syrian Civil Defense group extinguishing a store during airstrikes and shelling by Syrian government forces. AP Eastern Ghouta bombings: Syrian war in pictures A wounded 12-year-old Syrian boy, cries as he receives treatment at a make-shift hospital. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures Syrians carry a wounded man. AFP/Getty Eastern Ghouta bombings: Syrian war in pictures An injured man covered with blood at a medical point. Reuters Eastern Ghouta bombings: Syrian war in pictures People sit a medical point in the besieged town of Douma, Eastern Ghouta. Reuters Eastern Ghouta bombings: Syrian war in pictures Syrian Civil Defense running to help survivors. AP Eastern Ghouta bombings: Syrian war in pictures Injured children receive medical treatment. EPA

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 Douma on Saturday, which Syrian activists and rescuers say killed 40 people.

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 Isis in Syria and Iraq.

On Monday Mr Trump suggested 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 Mr Assad from further use of chemical agents. So the three countries are discussing a range of options, including preventing the Syrian president from conducting future attacks by striking military capabilities involved in carrying out such attack, the official said.

Asked whether France would take military action, Mr 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.

After Mr Trump spoke by phone with Theresa May, a 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."

Mr 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 America's 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 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.