It has been called the biggest nuclear threat to world safety since the 1962 Cuban missile crisis: as Vladimir Putin seeks to salvage his invasion of Ukraine, the Russian president has stepped up his threats to use nuclear weapons.
He said last week he would use "all available means" to keep Russia safe after unilaterally proclaiming four eastern Ukrainian provinces part of Russia. The US had "created a precedent" when it dropped two atomic bombs on Japan in 1945, he added.
At the weekend, Chechen leader and Putin loyalist Ramzan Kadyrov suggested the Russian president consider using "low-yield nuclear weapons" in Ukraine.
But the proposal was dismissed by the Kremlin on Monday. Putin's spokesman Dmitry Peskov said that there were "no other considerations" apart from using nuclear weapons according to Russia's military doctrine, which permits their deployment if Russia is struck first or if the very existence of the state is at risk.
Western officials and military experts believe the risk that Putin will deploy nuclear weapons is low. But as Russian forces suffer military setbacks in south-eastern Ukraine, they also recognise that risk is rising.
Here is what we know about the nuclear weapons Putin could be tempted to use:
'Tactical' vs 'strategic' nukes
The Cuban missile crisis was about "strategic" nuclear weapons, which are powerful enough to obliterate entire cities thousands of miles from any battlefield. The issue in Ukraine instead revolves around smaller, so-called tactical nuclear weapons.
These smaller nuclear warheads are intended for battlefield use and are designed to destroy targets in a specific area. Even so, many of the warheads are more powerful than the atomic bomb dropped on Hiroshima by the US, which had an explosive yield equivalent to about 20 kilotons of TNT.
"So-called tactical nuclear missiles for battlefield use have a yield of generally between one and 50 kilotons [of TNT] ... devastating over areas typically two square miles," General Sir Richard Barrons, former head of UK joint forces command, told the BBC on Monday.
The US and USSR once kept huge stockpiles but after the cold war ended the US gave up all but 230 of them, believing that "increasingly efficient conventional weapons could do the job" better instead, according to a 1989 analysis published by the Bulletin of the Atomic Scientists.
Russia kept about 2000 tactical nuclear warheads. They can be placed on various systems used to deliver conventional explosives, such as Kalibr cruise missiles or Iskander ballistic missiles, and can be launched from land or sea.
How would these weapons be deployed?
Experts see three ways in which Russia might use tactical nuclear weapons.
The first is demonstrative — a nuclear shot that does not kill anyone. It could be a detonation underground, over the Black Sea, perhaps somewhere high in the skies above Ukraine or on an uninhabited site such as Snake Island.
The blast's electromagnetic pulse would fry unprotected electronic equipment and the radioactive fallout, while large initially, would reduce to about 1 per cent of the initial radioactive blast in 48 hours. Most of the radioactive dust sucked into a rising cloud by the explosion would settle back on earth nearby within 24 hours of the strike and could be an extreme biological hazard. Other particles could be dispersed by prevailing winds and settle in much lower concentrations over large parts of the globe.
Even a demonstrative attack would start the "escalation ladder" and raise the prospect of a Russian attack on a big city. It would probably spark a global backlash for no military gain, making Russians "more of a pariah in the world than they ever have been", as US President Joe Biden said in a September 16 interview.
The demonstration effect may also be unclear as it would show that Russia was ready to break the taboo on nuclear weapons but that it remained cautious about using their explosive power to the full.
That, according to Sir Lawrence Freedman, professor emeritus of war studies at King's College London, is one reason the US discarded the demonstration option in 1945 before dropping a nuclear bomb on Hiroshima.
The second possibility is a strike on a Ukrainian military objective or key piece of infrastructure — for example, a missile strike on Zaporizhzhia nuclear power plant.
How useful this would be is also open to question. Ukraine's military forces are highly dispersed and US Army studies have concluded that a one kiloton warhead has to detonate within 90 metres of a tank to inflict serious damage.
Some experts say it would make little sense for Russia to strike battleground targets in provinces it now considers its own. Russia's poorly motivated and badly equipped army would also be exposed to the radioactive fallout.
The third and most escalatory move would be a strike on a Nato member, including the US — as suggested by Dmitri Trenin, the former head of the Carnegie Moscow Centre, a Russian think-tank.
In a state television interview last week, Trenin said Russia needed to show it was serious about a US nuclear strike for its deterrent to be effective. He added that the West was also wrong to assume that Putin would respond to battlefield defeats by using nuclear weapons only against Ukraine.
"It's entirely possible the strike would not hit the theatre of battle but somewhere a certain distance away," Trenin said.
How would Nato react?
How the West would respond to an attack on Nato is hard to predict. Article V invoking a collective defence response from other Nato members would be triggered. Moscow would risk a devastating nuclear retaliatory strike by the US.
Jake Sullivan, Biden's national security adviser, said last month that any Russian nuclear use would have "catastrophic consequences" without specifying what those consequences might be. He also made clear that the US had "spelt out" in private conversations with Moscow how the West would react. On Sunday, Nato secretary-general Jens Stoltenberg warned of "severe consequences for Russia".
That might involve a conventional military attack that destroys Russia's Black Sea fleet, as retired CIA director and army general David Petraeus suggested on Sunday.
But Western officials have by and large remained vague in their threats of retaliation even if Putin targets Ukraine, a non-Nato member, as deterrence rests on ambiguity.
Written by: John Paul Rathbone, Max Seddon, Ben Hall and Henry Foy
© Financial Times