President Vladimir Putin's order to place Russia's strategic forces on high alert has served as a stark reminder that he commands what experts agree is the world's biggest nuclear arsenal.

The move came on Sunday, four days after Moscow launched the invasion.

In it, Putin announced that he had ordered his military chiefs "to put the deterrence forces of the Russian army into a special mode of combat service".

On Monday, the United States said it still had not seen any "muscle movements" following Putin's weekend announcement that he was putting his nuclear forces on high alert, a senior US defence official said.

But some former US officials and experts caution that it would be a mistake to write off Putin's remarks as bluster, given the risk that Putin could decide to escalate to using nuclear weapons if he feels cornered over the war in Ukraine or if the war spills over into NATO.

Russia's defence ministry on Monday said its nuclear missile forces and Northern and Pacific fleets had been placed on "enhanced" combat duty, in line with an order the previous day from Putin.

The phrase special, or enhanced, combat duty appears to have stumped the Pentagon.

"It's not a term of art in what we understand to be Russian (nuclear) doctrine," the official said, speaking on condition of anonymity. "So that's why we're analysing it and reviewing it to try to understand what exactly it means."

Leaders of the US military, which built much of its intelligence collection architecture to spy on the Soviet Union, were not aware of Putin's decision until he made it publicly and, so far, there have not been big movements of weapons or forces to demonstrate what it means, the US official said.

"I don't believe we've seen anything specific as a result of the direction that he gave, at least not yet, in terms of appreciable or noticeable muscle movements," the official said.

The United States closely monitors everything from Russian nuclear storage facilities to deployments of nuclear-capable bombers, missile forces and submarines.

The White House said it saw "no reason to change" its nuclear alert levels at this time, and President Joe Biden assured Americans that they should not fear nuclear war.

The United States and Russia account for more than 90% of the world's nuclear weapons, but only a fraction of them are deployed, according to the Union of Concerned Scientists.

Here is a snapshot of Russia's nuclear capabilities.

The most warheads

Russia owns the greatest number of nuclear warheads of any country, although the United States has more deployed, or immediately usable, warheads.

A peace research institute in Stockholm, the SIPRI, has counted 6,255 Russian warheads against 5,550 for the US. China follows far behind with 350 and France with 290.

These figures, although widely accepted, are nevertheless estimates, notably because not all nuclear-capable weapons systems actually carry nuclear warheads.

According to the Nobel peace prize-winning International Campaign to Abolish Nuclear Weapons (ICAN), Russia spent $8 billion in 2020 on the building and maintenance of its nuclear arsenal.

Chain of command

The Russian constitution gives the president control over nuclear weapons, but the transmission of any order to use them, and the authentification of that order, also involve the defence minister and the armed forces' chief-of-staff.

How such a scenario would play out exactly "we don't know", said Pavel Podvig, a Russian independent military expert.

The two subordinates have "no right to veto, but there is still some kind of deliberation process," he said, adding: "It's not like there is a button on the president's table, there is a procedure."

Even if a nuclear strike order should come down, there is still the question of whether the armed forces would comply.

"They are not mad and they are not sectarians," said Pavel Luzin, a Russian military expert based in Moscow at Riddle, a think tank.

Many Russian government officials may well agree with western analysts who say that Russia would have much more to lose than to gain if it unleashed nuclear war.

"I personally don't think the Russian military elite will cheer at the idea of limited nuclear use in or about Ukraine," tweeted Kristin Ven Bruusgaard at Stanford University's Center for International Security and Cooperation.

"But who will break the news to Putin that this may in fact not work?", she added.

Doctrine and reality

Vladimir Putin in 2020 adopted a doctrine for the possible use of nuclear weapons, experts Hans Kristensen and Matt Korda recalled in last week's issue of the 'Bulletin of the Atomic Scientists'.

According to the doctrine, there are four acceptable justifications for going nuclear:
A ballistic missile attack against Russia or an ally,
the use of a nuclear weapon by an enemy,
an attack on a Russian nuclear weapons site,
or any attack threatening the existence of the state.

They quoted Putin as saying that Russia - which claims it has modernised close to 90% of its nuclear arsenal -- could never allow itself to "stand idle" in the face of potential enemies.

"You stop for one second and you start falling behind immediately," they quoted Putin as saying.

They added that Russian thinking may already have gone beyond the published doctrine.

"For example, officials explicitly threatened to use nuclear weapons against ballistic missile defence facilities, and in regional scenarios that do not threaten Russia's survival or involve attacks with weapons of mass destruction," they said.

"The real doctrine," they added, "goes beyond basic deterrence and toward regional war fighting strategies, or even weapons aimed at causing terror."