American military action against the Syrian government could lead to a worst-case scenario of a Russia-US conflict, Russian-instigated unrest on NATO's borders in Europe and the creation of a "Libya on steroids" after the downfall of Bashar al-Assad, analysts say.

Donald Trump on Wednesday said the Syrian president had crossed "beyond a red line" with a chemical attack on the Idlib town of Khan Sheikhun on Tuesday. Trump's vice-president, Mike Pence, said "all options" were on the table for a US response.

"What Assad did is terrible. What happened in Syria is truly one of the egregious crimes," Trump said while travelling to Florida aboard Air Force One.

But analysts and military experts told Middle East Eye on Thursday that any sort of wide-scale military action was almost impossible because of the Russian presence in Syria, could turn the country into a bona fide failed state and spark retaliatory confrontations by Russia beyond Syria's borders.

Nicholas Drummond, a defence and security analyst and former British army officer, said Trump would have three military options: precision air strikes using cruise missiles, bombers or strike aircraft to neutralise Syria's military capability, a Marine Corps task force on the ground to take control of strategic areas and physically remove Assad or a special forces mission to kill or capture the Syrian president.

Justin Bronk, a research fellow at the Royal United Services Institute in London, said that while the US had "myriad" military options, any action could spark unintended and far-reaching consequences.

Bronk, an airpower and technology specialist, said: "The US has air platforms, special forces, intelligence gathering and targeting all over Syria, so in that sense, they have a very good idea of things to target with should they wish."

But he said only a few assets, such as F-22 stealth bombers and B-2 strategic bombers, would be able to carry out strikes without detection, and would face advanced Russian forces already protecting Assad.

A US B-2 stealth bomber is serviced in its hangar (AFP)

Yes we can, but should we?

"The Russians have very capable surface-to-air missiles - S-300s and S-400s - at their air base in Latakia and their naval base at Tartus, as well as mobile assets. Those have ranges out to 250km, potentially even further depending on what missiles they're loaded with. They are very difficult to jam, and extremely high-end threats.

"Then there are Russian air superiority aircraft in Latakia, including high-end Sukhoi 30 and Sukhoi 35 fighters."

The probable US targets would be arms depots, and Syrian air force assets on Syrian air fields, "but again Russian specialists would potentially be at those sites, and there are huge potential issues involved with going after those targets".

"In other words, really high-end US stealth assets could probably conduct limited strikes with a fairly low risk of Russian engagement.

"But there are very few assets in the Western coalition that could conduct strikes on Syrian government targets without Russian permission. They would be operating in a very crowded airspace, flying through small corridors, and so it would be risky using scarce high-end assets for an uncertain military impact."

You would be trying to enforce a no-fly zone within range of Russian assets. Without total Russian commitment, that would be pretty unenforceable - Justin Bronk

Bronk added that the option of enforcing a no-fly zone to ground the Syrian air force and prevent further chemical attacks was itself laden with problems.

"Again, without Russia's total cooperation on that, which I believe is almost completely impossible without some sort of bargain on a number of other things, then you would be trying to enforce a no-fly zone within range of Russian assets, against a Syrian air force that operates the same aircraft as the Russians in the same theatre.

"There would be a huge risk of enforcement, for instance shooting down Russian assets, so without total Russian commitment that would be pretty unenforceable."

Bronk said he believed there would be every likelihood of retaliation from Russia.

"If America were to go for some sort of military action against Russian objections - call their bluff as it were - I don't think the Russians would actually fire on US aircraft unless they were directly attacked. I am sure they would take great pains not to.

"But at the same time, Russia has a long history of escalating horizontally rather than vertically, when confronted, so what you would expect to see is a whole lot of trouble flare up elsewhere whether that be Georgia, Ukraine, provocations around the Baltics - in other words, there would be a response."

Pro-Russian fighters in Ukraine (Reuters)

'Libya on steroids'

Another issue was the long-term consequences to Syria, said both Bronk and Drummond. Whatever Trump did, he would need a plan for rebuilding the country.

"The problem comes in with any desired end-state - what are the US moving towards? An all-out decapitation attempt to remove Assad would lead almost immediately to a Libya on steroids - the creation of chaos and pressure to do something about it," said Bronk.

"You look at the potential outcomes and none of them look good. If NATO or a coalition hit the Assad regime in 2013, you would have another Libya and they would have been blamed for the total chaos that Syria would still have now. That is the fundamental reason the strikes never went ahead under Obama."

He said a third option of stepping up support for anti-Assad rebel groups could also potentially aid groups such as the Islamic State (IS).

"If you were to try to intensify the pressure on Assad very quickly, that would leave the West open to exploitation in the messaging sphere, as in 'now you're creating more ungoverned space which Daesh (IS) would expand into - basically working for the terrorists'. You would be absolutely certain Russia would take that line."

Drummond said: "The danger of air strikes not supported by ground forces is reaping wholesale destruction that leaves the kind of vacuum that occurred in Iraq. This could open the door to IS or a regime worse than Assad."

Funeral of a Libya Dawn militiaman in Tripoli, Libya (AFP)

The art of the deal

And even in the best-case scenario, getting permission from Russia to bomb Assad would come with all sorts of concessions elsewhere, said Bronk.

"Russia has staked an enormous amount of its international reputation on being the defender of Assad, which sends a message to potential future partner states that they can keep you in power even if the West is all over you.

"Russia is working from a position of geopolitical strength. If it were to be talking about some sort of deal in return for abandoning Assad, it would likely demand enormous concessions outside Syria... on things like Ukraine, Crimea recognition, and demilitarisation of NATO forces in the Baltic states.

"So if you look at the potential gains versus what it would cost, no rational president who understood what the balance of power between NATO and Russia means, would ever take that sort of deal. That does not necessarily rule it out, but I think it is very unlikely.

"Without any sort of bargain with Russia, I don't think there is any good military option. I don't think there is any sort of possible outcome that one would want to be associated with long term."

Drummond said he thought it was likely that Trump would attempt to engage Putin directly to give Russia a chance to improve its standing among NATO allies.

"Indeed, either Trump or his foreign policy advisers have to syndicate any plan to avoid Russian casualties. If the US were to strike and injure Russian forces, this would only increase tensions in Europe. Trump could make a bad situation worse," he said.

If the US were to strike and injure Russian forces, this would only increase tensions in Europe. Trump could make a bad situation worse - Nicholas Drummond, security expert

"The ideal situation would be for the US to deploy sufficient force to remove Assad, neutralise IS, and stabilise the region. This could pave the way for a multinational UN-sanctioned peace keeping force to maintain safe zones while Syria is rebuilt. It would require a massive manpower and equipment commitment.

"There are a lot of variables in Syria that make predicting a likely course of military action difficult. Chief among these is Trump himself. He is very hard to read. But achieving a notable foreign policy success would help him immeasurably."

Bronk said, however, that Russia would carefully consider the impact of any retaliation to Trump's gambit. "Neither side wants a third world war [but] Russia is not going to take any kind of public defiance of its wishes without responding," he said.

But the Russians still do not know what they're dealing with in Trump, he added. "Unless you subscribe to the full-scale 'Manchurian Candidate' theory that he is basically an asset, which is pretty extreme, then the Russians don't really know what they are dealing with.

"There will be a caution on the Russian side. Fundamentally, they cannot afford to provoke a full-scale war. It would be more 'threshold' warfare - in terms of trying to irritate and humiliate NATO without ever actually going above the threshold.

"The problem is, unlike the Obama administration, there is very little way to know what the threshold with Trump is."