Russia's ambassador to Lebanon has said any US missiles fired at Syria would be shot down and the launch sites targeted, a step which could trigger a major escalation in the Syrian war.
Alexander Zasypkin, in comments broadcast on Tuesday evening, said he was referring to a statement by Russian president Vladimir Putin and the Russian chief-of-staff.
"If there is a strike by the Americans then... the missiles will be downed and even the sources from which the missiles were fired," he told Hezbollah's al-Manar TV.
The Russian military said on 13 March it would respond to any US strike on Syria, targeting any missiles and launchers involved in such an attack.
Mr Zasypkin added that a clash "should be ruled out and therefore we are ready to hold negotiations".
Moscow and Damascus deny the Syrian government was behind a suspected chemical weapons attack that has led to Donald Trump and Western allies considering military action against Bashar al-Assad's regime.
But the World Health Organization (WHO) said on Wednesday around 500 people were treated for "signs and symptoms consistent with exposure to toxic chemicals" following the attack on a rebel enclave near Damascus at the weekend.
:: In other developments ::
The WHO said more than 70 people sheltering from bombardment in basements in the former rebel-heald pocket of eastern Ghouta were reported to have died. Of those, 43 were "related to symptoms consistent with exposure to highly toxic chemicals," citing reports from its local health partners.
Responding to Mr Zasypkin's comments, General Sir Richard Barrons, who led the UK’s Joint Forces Command from 2013 to 2016, told BBC Radio 4’s Today programme: “I hope the ambassador has chosen his words very carefully because what he’s actually saying is that if the US and allies decide to strike against Syrian chemical weapons and delivery aircraft, not only are they going to try and shoot down the missiles in flight - which they're capable of doing, but won't be with total success - but by saying the words 'launch platforms', he’s saying they are going to try and sink ships, sink submarines and shoot aircraft out of the sky - that’s war.”
On Wednesday, Moscow attempted to dial down the rhetoric, saying it hoped all sides involved in Syria would avoid doing anything that could destabilise an already fragile situation in the Middle East.
It maintained allegations that Syrian government forces had carried out a chemical weapons attack were not based on real facts and said it wanted an impartial investigation into the incident.
Asked about Mr Zasypkin's comments, Kremlin spokesperson Dmitry Peskov said he did not want to comment on such matters.
"As before, we would like to hope that all sides will avoid any steps that a) are not provoked by anything and b) could significantly destabilise an already fragile situation in the region," he told reporters.
The prospect of military action against Mr Assad's government prompted European airspace authorities to warn planes to be careful when flying close to Syria.
The Eurocontrol airspace organisation said the European Aviation Safety Agency (EASA) had sent a "Rapid Alert Notification" that flight operations covering Cyprus and the surrounding skies needed to consider the possibility of air or missile strikes into Syria.
In a notice posted to Eurocontrol's website, EASA said: "Due to the possible launch of air strikes into Syria with air-to-ground and/or cruise missiles within the next 72 hours, and the possibility of intermittent disruption of radio navigation equipment, due consideration needs to be taken."
More than 20 flights are due to take off from the UK to Cyprus on Wednesday, including ones provided by British Airways, easyJet and Ryanair.
On Tuesday, Russia and the US blocked each other's attempts in the UN Security Council to set up international investigations into suspected chemical weapons attacks in Syria.
Each country placed a resolution before the security council to vote on. The US text was vetoed by Russia, while the Russian text was not adopted because it did not gain enough votes.
The US resolution had proposed an independent mechanism for investigating the 7 April attack in the town of Douma, eastern Ghouta. It also called for an “attribution” clause that would identify the perpetrator of the attack.
Russia, meanwhile, proposed that the UN secretary-general Antonio Guterres choose the investigators, but that the results be reviewed by Russia for “acceptance” prior to making them public, according to US Ambassador to the UN Nikki Haley. It also lacked any clause to assign attribution of the suspected attack.
"History will record that, on this day, Russia chose protecting a monster over the lives of the Syrian people," Ms Haley said, referring to Mr Assad.
She said the US "went the extra mile" to get Russian support for the resolution to ensure that a new investigative body would be impartial, independent and professional - provisions she said were not in the rival Russian resolution.
Vassily Nebenzia, Russian ambassador to the UN, accused the US of trying to mislead the international community, and said it was "taking one more step towards confrontation".
He said the US and its allies did not need a resolution to determine responsibility because it was already blaming Syria and Russia.
"You do not want to hear the fact that no traces of a chemical attack were found in Douma," Mr Nebenzia said. "You simply have been looking for a pretext" and want the resolution to fail "to justify the use of force against Syria".
Its foreign ministry said it would help the Organisation for the Prohibition of Chemical Weapons (OPCW) in a fact-finding mission into the alleged attack, which opposition activists say killed 40 people.
The White House announced on Tuesday Donald Trump would skip an upcoming summit in South America and instead remain in the US to "oversee the American response to Syria and to monitor developments around the world”.
The US president promised to respond "forcefully" to the alleged chemical attack, and that the US had "a lot of options militarily".
“We can’t let atrocities like we all witnessed... we can’t let that happen in our world... especially when we’re able to because of the power of the United States, the power of our country, we’re able to stop it,” he said.
Theresa May joined Mr Trump in calling for a response to the Syrian regime's latest use of chemical weapons, "if confirmed".
In separate phone calls, Ms May, Mr Trump and French president Emmanuel Macron agreed the international community should work together to hold Mr Assad's government and its backers to account.
Of the calls, a Downing Street spokesperson said: “They agreed that reports of a chemical weapons attack in Syria were utterly reprehensible and, if confirmed, represented further evidence of the Assad regime’s appalling cruelty against its own people and total disregard for its legal obligations not to use these weapons.
“They agreed that the international community needed to respond to uphold the worldwide prohibition on the use of chemical weapons. They agreed they would continue working closely together and with international partners to ensure that those responsible were held to account.”
Dozens of buses carrying hundreds of rebel fighters, along with family members and civilians who did not wish to come back under Assad’s rule, left Douma for opposition areas near Aleppo on Tuesday, the Syrian Observatory for Human Rights said.
The deal restores Mr Assad’s control over the whole of eastern Ghouta – formerly the biggest rebel bastion near Damascus.
As part of the surrender deal, the Jaish al-Islam group that controlled the town released scores of people it had been holding.
Additional reporting by agencies
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