When Russia first invaded Ukraine, President Vladimir Putin cautioned the West about his nuclear capabilities, ordering his nuclear forces be put on high alert.
At the time, UN Secretary-General Antonio Guterres said "nuclear conflict, once unthinkable, is now back within the realm of possibility".
More than seven months since the invasion began, as Russia struggles to maintain control of its captured territories against Ukrainian forces, Mr Putin has upped the stakes once again.
Russia plans to annex areas of Ukraine it currently holds by holding referendums asking residents whether they want to join Russia.
The West says this is a violation of international law, with Poland's president Andrzej Duda saying the result would be "decided in the Kremlin and not by voting".
Despite the referendums being treated as a farce, treating parts of Ukraine as Russian territory could give Mr Putin the justification he needs to escalate his agenda.
In a televised national address last week, Mr Putin said he would defend Russian territory with all means at his disposal.
"If the territorial integrity of our country is threatened, we use all available means to protect our people — this is not a bluff," Mr Putin said.
What are Russia's nuclear capabilities?
Russia is one of the most dominant nuclear forces in the world.
The Federation of American Scientists (FAS) — a nonprofit research and advocacy group — estimates Russia has 5,977 nuclear warheads, more than any other country (though 1,500 are categorised as 'retired' and awaiting dismantlement).
Of these, 1,588 are deployed and ready for use. The United States has 1,644.
Russia's missiles can be fired from land, from submarines and from aeroplanes.
How is Russia's nuclear arsenal arranged?
As of early 2022, FAS estimates that Russia had a stockpile of approximately 4,477 nuclear warheads ready for use, assigned for long-range strategic launchers and shorter-range tactical nuclear forces.
Of these, about 1,588 strategic warheads are deployed on ballistic missiles and at heavy bomber bases, while about 977 additional strategic warheads, along with 1,912 non-strategic warheads, are held in reserve.
FAS estimates Russia has 306 nuclear-armed intercontinental ballistic missile launchers — some which have a range up to 16,000 kilometres — located across its 11 divisions.
The largest of these divisions is Tatishchevo, which is the base for 60 ICBM launchers.
Will Russia actually use its nuclear weapons?
It's impossible to say for sure what Mr Putin will or will not do.
It was only in August that he wrote to the Tenth Review Conference for the Treaty on the Non-Proliferation of Nuclear Weapons that there "can be no winners in a nuclear war and it should never be unleashed".
His words then seemed to strike a reassuring note, portraying Russia as a responsible nuclear power.
Russia also extended its New START treaty with the US in March last year.
Due to expire in 2026, the treaty caps the number of strategic nuclear warheads each country can deploy, and limits the land and submarine-based missiles and bombers to deliver them.
Jessica Genauer, international relations lecturer at Flinders University, said the possibility of Russia using its nuclear weapons could not be discounted but it would be highly unlikely.
Dr Genauer said Mr Putin knew that using nuclear weapons would be a severe escalation and would be met in kind by Ukraine's Western allies.
"Putin has been drawing on the threat of nuclear weapons almost since the start of Russia's full-scale invasion, ever since it became clear that Russia's invasion of Ukraine would not be a short and decisive military victory for Russia," she said.
"The recent announcement of a 'partial military mobilisation' in Russia, makes it clear that Putin has chosen to double down on Russia's invasion of Ukraine, rather than try to find an exit strategy.
"Given Russia's already weakened military state, and the breadth of sanctions already being applied, it seems unlikely that Putin would want to risk the response from Western partners to a nuclear strike."
Still, many say Mr Putin's threats should be taken seriously.
European Union foreign policy chief Josep Borrell said now was a "dangerous moment" because the Russian army "has been pushed into a corner".
Mr Borrell said Mr Putin's nuclear threats must be taken seriously, and has called on the Russian president to negotiate a peace deal.
Until then, he said the EU would continue to put the pressure on Russia.
ABC/wires
