Airlines have been warned of the danger from possible airstrikes and missile attacks on Syria, as Western allies prepare a military response to a suspected chemical weapons attack in the country, said media reports.

In a message on Eurocontrol's website, the European Aviation Safety Agency (EASA) wrote: "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."

The potential threat to carriers has grown on a day of rising tensions and threats as the US, Britain and France seek to co-ordinate their response to last week's suspected chemical attack which killed 70 people in the rebel-held town of Douma.

Early today, President Trump put Syria and Russia on notice in a Twitter post, promising that missiles fired at Syria “will be coming, nice and new and ‘smart!’” and telling the Kremlin that it should not partner with a “Gas Killing Animal who kills his people and enjoys it!”

Defense Secretary Jim Mattis said on Wednesday that the United States is still assessing the intelligence on the suspected chemical attack, but that military planning was proceeding.

“We stand ready to provide military options if they’re appropriate, as the president determined,” he said.

Amidst the sudden turn of events, Rusisian President Putin called up Israeli Prime Minister Benjamin Netanyahu, urging Israel not to take any action to destabilise Syria and threaten its security.

The sabre-rattling began when Alexander Zasypkin, the Russian ambassador in Beirut, told Hezbollah's al-Manar TV: "If there is a strike by the Americans, then...the missiles will be downed and even the sources from which the missiles were fired".

His remarks, raising the possibility of a US-Russian confrontation, provoked a belligerent response from President Donald Trump, who tweeted: "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 shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!," tweeted Trump.

The World Health Organization said on Wednesday that there were reports of about 500 people in Douma who have symptoms similar to people exposed to toxic chemicals.

It said about 70 people had died while taking shelter in basements and 43 of them had signs of being exposed to “highly toxic chemicals.”

He added: "Our relationship with Russia is worse now than it has ever been, and that includes the Cold War. There is no reason for this."

Meanwhile, US Defence Secretary James Mattis said America was still assessing intelligence on the suspected chemical attack in Syria.

US military officials revealed that the Syrian military had started re-positioning some air assets ahead of possible strikes.