Russian President Vladimir Putin has broadened the remit of his nuclear doctrine to fend off Western-supported attacks in the Ukraine war, threatening to use nuclear weapons against non-nuclear-armed nations even if not attacked by them.

During a meeting with Russia's Security Council on Wednesday, local time, he outlined three key changes to the Kremlin's official nuclear doctrine — signed in 2020 — as a response to ongoing deliberations in the United States and Britain about giving Ukraine permission to fire long-range missiles into Russian territory.

Mr Putin said under the lowered threshold, Russia could deploy nuclear bombs even if it was struck with conventional weapons, and that Moscow would consider any assault on it supported by a nuclear power to be a "joint attack". 

Russia reserved the right to also use nuclear weapons if it or ally Belarus were the subject of aggression, including by conventional weapons, he added.

The 71-year-old, who is the primary decision-maker on Russia's vast nuclear arsenal, said he wanted to underscore one key change in particular.

"It is proposed that aggression against Russia by any non-nuclear state, but with the participation or support of a nuclear state, be considered as their joint attack on the Russian Federation," Mr Putin said in his opening remarks to the council.

He said the new rules would be effectuated if Russia detected a large-scale launch of enemy missiles, aircraft or drones was coming its way: "The conditions for Russia's transition to the use of nuclear weapons are also clearly fixed."

Mr Putin said the clarifications were carefully calibrated and commensurate with the modern military threats facing Russia — confirmation that the nuclear doctrine was changing.

The implications

Russia's warning to the West comes amid Ukrainian pleas to fire long-range weapons (many already in its possession) into Russia, including British Storm Shadows and American ATACMS ballistic missiles.

Ukrainian President Volodymyr Zelenskyy argues these will help hit vital military depots with precision, but the US has treaded carefully, fearing such a move would escalate the war and pit NATO in direct conflict with Russia. 

Russia has previously provoked war with NATO, accusing the US and European nations of de-facto participation in the conflict, but has not come as far as spelling out changes to the use of its nuclear arsenal. Earlier this month it said it was considering updating the doctrine.

With Ukraine losing key towns to gradually advancing Russian forces in the country's east, the war is entering what Russian officials say is the most dangerous phase to date.

Western aid for Kyiv has remained steady, with the US pledging a further $375 million in aid on Thursday, although it is unclear what bearing Mr Putin's nuclear threat will have on considerations of long-range weapon restrictions. 

Many view Russia brandishing its nuclear saber as little more than a bluff, but some analysts say it is "because of and not in spite of" the fact that Moscow has repeatedly held its nuclear arsenals over Western heads that leaders should take these threats seriously.

Russia's current published nuclear doctrine, set out in a 2020 decree by Mr Putin, says it may use nuclear weapons in case of a nuclear attack by an enemy or a conventional attack that threatens the existence of the state.

The innovations outlined include a widening of the threats under which Russia would consider a nuclear strike, the inclusion of ally Belarus under the nuclear umbrella and the idea that a rival nuclear power supporting a conventional strike on Russia would also be considered to be attacking it.

Nuclear armed states that could be drawn into this include France, the United Kingdom, Israel and most crucially the US, which along with Russia controls 90 per cent of the world's nuclear warheads. 

China, Pakistan, India, and North Korea possess the remainder, but neither has actively been involved in the Ukraine war. A further 32 states also either host nuclear weapons or endorse their use.

Ukraine is neither a nuclear state, nor is a part of NATO, but is backed by the alliance.

How has Russia's threat been received?

Mr Zelenskyy has urged the West to disregard Russia's so-called "red lines", and some Western allies have also urged the US to do just that.

"Russia no longer has any instruments to intimidate the world apart from nuclear blackmail," Andriy Yermak, Mr Zelenskyy's chief of staff, said in response to the Russian president's Wednesday remarks. 

"These instruments will not work."

Mr Putin, who casts the West as a decadent aggressor, and US President Joe Biden, who casts Russia as a corrupt autocracy and Mr Putin as a killer, have both warned that a direct Russia-NATO confrontation could escalate into World War III. Republican presidential candidate Donald Trump has also warned of the risk of nuclear war.

In his comments to Russia's Security Council, a type of modern-day politburo of Mr Putin's most powerful officials including influential hawks, he said work on amendments on changing the doctrine had been going on for the past year.

"The nuclear triad remains the most important guarantee of ensuring the security of our state and citizens, an instrument for maintaining strategic parity and balance of power in the world," he said.

Russia, he said, would consider using nuclear weapons "upon receiving reliable information about the massive launch of aerospace attack vehicles and their crossing of our state border, meaning strategic or tactical aircraft, cruise missiles, drones, hypersonic and other aircraft".

No explicit laws restraining nuclear weapons use

Casualties from a nuclear war between Russia and the US could reach tens of millions, with even a single bomb having the capacity to wipe out about 580,000 people, according to estimates from the International Campaign to Abolish Nuclear Weapons. 

US-Russian arms control agreements also classify "tactical" nuclear weapons, which are presented as having smaller yield, and are intended for battlefield use as opposed to strategic weapons fired across vast distances.

But even these smaller warheads are hugely destructive and comparable in strength with the two atomic bombs dropped by the US in Japan during World War Two which killed about 210,000 people.

In 2022, Washington was so concerned about the possible use of tactical nuclear weapons by Russia that it warned Mr Putin over the consequences of using them, according to Central Intelligence Agency Director Bill Burns.

The two-and-a-half year Ukraine war has triggered the gravest confrontation between Russia and the West since the 1962 Cuban Missile Crisis — considered to be the closest the two Cold War superpowers came to intentional nuclear war.

Internationally, little power exists to prevent nuclear powers who invoke the use of their arsenal.

The UN's Treaty on the Prohibition of Nuclear Weapons does formally push for nuclear disarmament and outlaws the development, testing, production, acquisition, possession and stockpiling of nuclear weapons but neither Russia nor the US are a part of it.

Netherlands is also the only NATO member participating in the treaty.

ABC/Reuters