U.S. President Donald Trump warned Russia on Wednesday of imminent military action in Syria over a suspected poison gas attack, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar al-Assad.

Trump was reacting to a warning from Russia on Tuesday that any U.S. missiles fired at Syria over the deadly assault on a rebel enclave would be shot down and the launch sites targeted.

His comments raised fears of direct conflict over Syria for the first time between the two world powers backing opposing sides in the country's protracted civil war, which has aggravated instability across the Middle East.

"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!'" Trump wrote in a post on Twitter.

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? —@realDonaldTrump

In response, Russia's foreign ministry said in a Facebook post that "smart missiles should fly toward terrorists, not toward the lawful government."

Foreign ministry spokesperson Maria Zakharova said any U.S. missile salvo could be an attempt to destroy evidence of the reported gas attack in the Syrian town of Douma, for which Damascus and Moscow have denied any responsibility.

The chief of Russia's armed forces, Valery Gerasimov, made it clear that Russia wouldn't hesitate to try to shoot down U.S. cruise missiles and target the "platforms" from which they were launched.

Those "platforms" would be U.S. destroyers such as the USS Donald Cook, stationed off the Syrian coast.

Putin advises 'common sense'

However, other statements by Russian officials Wednesday suggest the political leadership in Moscow may be trying to dial down the rhetoric to provide some wiggle room to react in case of a strike.

Speaking to a group of newly minted foreign ambassadors in Moscow, Russian President Vladimir Putin alluded to the crisis only briefly, saying he hoped "common sense" would prevail.

His chief spokesperson, Dmitry Peskov, said, "Hopefully, all countries will refrain from steps that, on the one hand, would be totally unwarranted and on the other hand, may exacerbate the already fragile situation in the region."

In Douma, dozens of inhabitants died and hundreds were injured in the attack, according to the World Health Organization (WHO).

U.S. assessing information

U.S. Defence Secretary Jim Mattis, striking a cautious tone after Trump's threat of missile strikes, said the United States was assessing intelligence about the suspected attack.

Asked if he had seen enough evidence to blame Assad, Mattis said: "We're still working on this."

The U.S. military is ready to provide military options, if appropriate, he added. It was unclear if his remarks reflected unease about Trump's apparent move toward military action.

Two U.S. government sources told Reuters the United States still did not have 100 per cent solid evidence of what nerve agent was used in Syria and where it came from. However, there is some evidence it was sprayed from helicopters, they said.

After Trump's tweet, the Syrian Observatory for Human Rights — a British-based war monitor with a network of sources on the ground — reported that pro-government forces were emptying main airports and military air bases.

The Syrian military has also been repositioning some air assets to avoid the fallout from possible missile strikes, U.S. officials told Reuters.

The Russian military said later it had observed movements of U.S. naval forces in the Gulf. Any U.S. strike would probably involve the navy in waters within range of Syria, given the risk to aircraft from Russian and Syrian air defence systems.

The Syrian foreign ministry accused the United States, which has supported some rebel groups in Syria's conflict, of using "fabrications and lies" as an excuse to hit its territory.

"We are not surprised by such a thoughtless escalation by a regime like the United States regime, which sponsored terrorism in Syria and still does," the state news agency SANA cited an official source in the ministry as saying.

Talking about timing

After the Douma attack, the insurgent group dug in there, Jaish al-Islam, finally agreed to withdraw. That clinched a huge victory for Assad, crushing a long rebellion in the eastern Ghouta region near Damascus.

In London, British Prime Minister Theresa May said all the indications pointed to Syrian government responsibility for the Douma attack and such "a shocking and barbaric act" could not go unchallenged.

"We're rapidly reaching an understanding of what happened on the ground," she told reporters. "We'll be working with our closest allies to consider how we can ensure that those responsible are held to account."

The BBC reported later that May was ready to give the go-ahead for Britain to take part in military action. She would not seek approval from Parliament, the BBC said, despite calls from the opposition Labour Party for Parliament to be given a say.

A spokesperson for May declined to comment on the BBC report.

Attack kills dozens in Douma, but government denies involvement 1:11

Oil prices jumped to their highest level in more than three years on Wednesday after Trump's missile warning, and U.S. stock index futures fell sharply amid alarm over possible Russian-U.S. conflict over Syria.

White House officials did not immediately provide any more detail about Trump's remarks. The U.S. Defence Department said it did not comment on "potential future military actions."

500 treated from chemical weapons

The World Health Organization said on Wednesday that 43 people had died in Saturday's attack on Douma from "symptoms consistent with exposure to highly toxic chemicals," and more than 500 in all had been treated.

The WHO cautioned that it has no formal role in forensic inquiries into the use of chemical weapons. International inspectors are seeking clearance from Damascus to visit Douma under safe conditions to determine whether globally banned munitions were used, though they will not assign blame.

A man walks with his bicycle past damaged buildings in the besieged town of Douma, eastern Ghouta, on March 30. (Bassam Khabieh/Reuters)

Moscow and Washington blocked attempts by each other at the United Nations Security Council on Tuesday to set up international investigations into chemical weapons attacks in Syria.

Moscow's threat to down U.S. missiles came from its ambassador to Lebanon, Alexander Zasypkin, who said it was based on previous statements by Putin and the Russian armed forces chief of staff.

The Russian military said on March 13 that it would respond to any U.S. strike on Syria by targeting any missiles and launchers involved. Russia is Assad's most powerful ally, and its devastating air power has helped him wrest back large areas of territory from rebels since 2015.

Zasypkin also said a clash between Russia and the United States over Syria "should be ruled out and therefore we are ready to hold negotiations."

Airlines warned

With tensions growing, however, pan-European air traffic control agency Eurocontrol warned airlines to exercise caution in the eastern Mediterranean due to the possible launch of airstrikes into Syria over the next 72 hours.

Eurocontrol said that air-to-ground and cruise missiles could be used within that period and there could be intermittent disruptions of radio navigation equipment.

Both Russia and Iran, Assad's other main ally, have warned his enemies against military action in recent days, underlining their commitment to the Syrian government they have armed and supported through years of conflict.

Ali Akbar Velayati, the top adviser to Iranian Supreme Leader Ayatollah Ali Khamenei, said during a visit to Damascus on Tuesday that an Israeli attack on an airbase in Syria earlier this week would "not remain without response."

Israel held top-level security consultations on Wednesday amid concern it could be targeted by Syria or Iran if the U.S. strikes Syrian government forces.

Risk of 'uncontrollable escalation'

Syria's Russian-supplied air defences shot down an Israeli F-16 jet in February during a bombing run against what Israel described as Iranian-backed positions in Syria.

Last year, the U.S. carried out strikes from two navy destroyers against a Syrian airbase after another toxic gas attack on a rebel-controlled pocket.