WASHINGTON - US President Donald Trump on Wednesday warned that "missiles will be coming" in response to an alleged chemical atrocity in Syria, as Russia scrambled to deflect blame from its ally Bashar al-Assad.

With renewed US military action seemingly imminent - and as Trump threatened that the US missiles would be "nice and new and 'smart!'" - both Washington and Moscow raised the rhetorical stakes.

Trump's warlike tweet apparently came in response to a warning from Russia's ambassador to Beirut, who took to a television network run by the armed group Hezbollah to warn that any US missiles would be shot down "as well as the sources they were fired from."

If the US action follows the pattern of a previous punitive strike on Syria ordered by Trump last year it will begin with a salvo of cruise missiles fired from US warships in the Mediterranean. US Defence Secretary Jim Mattis said the Pentagon is ready to provide options for a Syria strike in response to a suspected chemical attack, but noted the US and its allies are still gathering information.

Meanwhile, Moscow said the formerly rebel-held district of Eastern Ghouta - including Douma, the target of Saturday's alleged chemical attack - had been "totally stabilized" and would soon be patrolled by Russian military police. But the Russian army continued to deny their side's latest victory came after Assad launched a chemical attack on the last rebel-held pocket of the enclave, instead accusing the White Helmets civil Defence organization of staging the massacre.

But, while his lieutenants continued to up the ante with threats and allegations, Russia's President Vladimir Putin adopted a more statesmanlike tone, in remarks to new ambassadors presenting their credentials at the Kremlin. "The state of the world cannot but provoke concern," he said. "The situation in the world is becoming more and more chaotic but all the same we hope that common sense will finally prevail and international relations will take a constructive path."

Moscow on Wednesday said the US should strike rebels rather than "the legitimate government" in Syria. "Smart rockets should be aimed at terrorists, rather than the legitimate government, which has for several years been fighting international terrorism on its territory," foreign ministry spokeswoman Maria Zakharova said in a Facebook post.

Syria’s army has evacuated key defence buildings in Damascus following intelligence they might be targeted in a Western strike in response to an alleged chemical attack, a monitor said Wednesday. “The buildings of the ministry of defence and the army headquarters have been empty for two days,” the Syrian Observatory for Human Rights monitoring group said.

Russia has threatened retaliatory action against the US if missiles are fired at Syria over chemical attack allegations it claims are fabricated. “If there is a US strike... the missiles will be downed as well as the sources they were fired from,” Russian Ambassador to Lebanon Alexander Zasypkin said late Tuesday. He made the comments on Al-Manar, a Lebanon-based television channel run by the Iran-backed Hezbollah movement, another key military ally of Syrian President Bashar al-Assad’s regime.

Russian President Vladimir Putin on Wednesday called on Israel’s Prime Minister Benjamin Netanyahu not to take any action that could further destabilise the situation in Syria, following Israeli strikes against a regime military base on Monday.

In a telephone call, Putin urged Netanyahu “to abstain from any action that could further destabilise the situation in the country, which would represent a threat to its security,” the Kremlin said in a statement.

Sweden, a non-permanent Security Council member, said Wednesday it opposes military strikes on Syria, suggesting such action would violate international law.

Trump's tweets were more belligerent - he told Russia "You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!" - and he admitted that US relations with Russia have plunged to a historic low. But he notably also said there was "no reason for this," reiterated his hope for talks with Putin to halt a new arms race, and blamed his domestic political opponents for poisoning ties.

Air traffic alert

Trump and other Western leaders have vowed a quick and forceful response to Saturday's alleged gas attack, which rescue workers say killed more than 40 people in the Damascus suburb of Douma. Efforts at the UN Security Council to find a diplomatic solution failed on Tuesday, with Washington and Moscow opposing each other's rival motions to set up an international investigation into chemical weapons use.

 The United States, Britain and France have argued the incident bears all the hallmarks of a strike ordered by the regime, which has been blamed for previous attacks by the Organisation for the Prohibition of Chemical Weapons (OPCW).

Both Trump and his Defence secretary Jim Mattis abruptly cancelled upcoming travel plans Tuesday, as the USS Donald Cook - a guided-missile destroyer - moved to within striking range of Syria.

The air traffic control agency Eurocontrol has also released an alert from the European Aviation Safety Agency to flight operators in nearby airspace of "the possible launch of air strikes into Syria with air-to-ground and/or cruise missiles within the next 72 hours."

French President Emmanuel Macron, who has been coordinating closely with Washington, said he would decide on a response "in the coming days."

"Our decision will not target allies of the regime or attack anyone but rather attack the regime's chemical capabilities," he said, insisting he did "not want an escalation."

As it looked to head off the threat of Western strikes, Syria said it had invited the OPCW to visit the site of the alleged attack in Douma, a town in the Damascus suburb of Eastern Ghouta that is on the verge of falling to the regime after a long and bloody siege.

The OPCW said it would "shortly" deploy a fact-finding team to Douma for an investigation, but US officials said they were working from their own information and would not necessarily hold back.

The World Health Organization on Wednesday demanded "immediate" access to the victims of the alleged chemical attack.

"We should all be outraged at these horrific reports and images from Douma," said Peter Salama, the UN agency's chief of emergency response.

"WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts, and to deliver a comprehensive public health response," he added.

Damascus agreed to hand over its chemical arsenal in 2013, narrowly avoiding American and French air strikes in retaliation for a suspected sarin attack.

That incident, which killed hundreds, also took place in Eastern Ghouta.