"The immediate priority is to avert the danger of war," Russian's ambassador to the UN, Vassily Nebenzia, told reporters following a closed-door meeting of the Security Council.

"I don't think we're at the Cuban missile crisis level, but we're getting pretty damn close," Russia expert Boris Zilberman of the Foundation for Defense of Democracies told AFP.

But the Syria conflict is a complex web of overlapping wars which have drawn in several world powers, and the risk of miscalculation is as high as Trump's Twitter rhetoric is provocative.

No serious observer thinks Washington or Moscow want to fight each other over Bashar al-Assad's latest alleged use of banned chemical weapons on his country's civil war battlefield.

President Donald Trump's threat of action against the Syrian regime places the United States and Russia at perhaps the greatest risk of conflict since the days of the Cold War.

Asked if he meant war between the United States and Russia, he said: "We cannot exclude any possibilities unfortunately."

Saturday's alleged gas attack on the then rebel-held Damascus suburb of Ghouta reportedly left more than 40 people dead, and dramatically raised the stakes for outside powers.

Trump, who unleashed a one-off US cruise missile salvo a year ago in response to a similar attack blamed on Assad's forces, reacted with fury and threatened a renewed intervention.

President Vladimir Putin's Kremlin has deployed warplanes and troops to Syria to defend Assad's regime, and -- as usual -- issued a series of conflicting denials on his behalf.

Missile salvo

By Wednesday, as Russian and Syrian forces took charge of the formerly rebel-held area that was bombed, Trump was explicitly threatening on Twitter to launch more missiles.

The White House is still in discussions with its allies France and Britain and with Trump's own military commanders -- but a strike still seems more than likely after UN talks failed.

Defense Secretary Jim Mattis, often a more cautious voice than Trump's, pointed towards a US response Thursday when he told lawmakers that "some things are simply inexcusable."

Last April, US warships in the Mediterranean fired cruise missiles at a Syrian air base. Russian forces were warned in advance and pulled out of the target area, which was rapidly repaired.

Then in February this year, US forces working alongside Kurdish militia in the east of Syria clashed with a regime force killed scores of Russians working as mercenaries.

"In Syria, a handful of weeks ago, the Russians met their match and a couple hundred Russians were killed," outgoing CIA chief Mike Pompeo boasted on Thursday, at a Senate hearing.

Trump, after initial reluctance to undermine his hope of warmer ties with Putin, has also imposed sanctions on Russian oligarchs and expelled 60 alleged spies under diplomatic cover.

But Syria's war has continued, along with chemical strikes, so most experts now suppose that any new US response must be broader, with perhaps more targets and more than one night of bombing.

Russia, at least in its public pronouncements, is also more determined not to back down. The Kremlin's ambassador to Beirut vowed that Russian forces would try to shoot down any US missiles.

The Russian military has also made belligerent statements, but Putin himself and the Moscow foreign ministry have instead played the inj

ured party, demanding proof of Assad's culpability.

Meanwhile, the world is holding its breath. Could military action from Washington and its allies trigger an armed confrontation with Putin's Russia, a resentful nuclear-armed foe?

"The big concern here is always mistakes, unintended consequences," said Zilberman. "Especially if they're considering hitting a wider range of targets than they did last time."

But, despite the Kremlin's stern words, few expect Russia to seek confrontation. At the weekend, Israeli jets hit a Syrian airbase housing Iranian forces and Russia did nor respond.

Nevertheless, experts say the Russian "red line" would be the safety of its own forces, which are based alongside their Syrian and sometimes Iranian comrades in many areas of western Syria.

"The situation is aggravated by multiple points of tension and repeated similar cases, in Syria as with Skripal," said Boris Toucas of the Center for Strategic and International Studies.

Sergei Skripal is a former Russian double-agent recovering in hospital in Britain after he was poisoned on March 4 by a nerve agent, in what London alleges was a Russian assassination bid.

"Nevertheless, none of the protagonists have any interest in a direct confrontation, as shown by the absence of any Russian reaction to last year's American strikes," Toucas argued.

Trump's response, if and when it comes, will have taken several days to organize and thus Russian forces may have been able to put some space between themselves and likely targets.

World order eroding

But, equally, there are reports from Syria that some regime forces may be gathering in Russian-protected bases for shelter.

Putin may also feel the need to react in order to protect his leadership of the Russia-Syria-Iran coalition.

"They've got to look tough, like they're not letting this happen," Zilberman said, predicting that Russia's aggressive attitude would continue and that US-Russia ties will only degrade further.

"Certainly for domestic consumption they've got to look like they're not rolling over for Israel, for the United States or for France or whoever else," he said.

But both Washington and Moscow suffer from credibility problems that could increase the chances of escalation.

The US administration is in turmoil, with vacancies or very new appointees in major national security positions and policy declared by Trump tweets sent in reaction to cable news segments.

"The resulting confusion is a problematic element of uncertainty, at a time when the post Cold War international order is eroding rapidly," warns Toucas.

Meanwhile, Putin's inner circle of senior officials and allied oligarchs has seen their wealth drop by billions of dollars over the past week since the latest round of US sanctions.