Russia has revamped its nuclear doctrine. Here's what that could ...

Russian President Vladimir Putin has signed a revised doctrine allowing Russia to use its nuclear capabilities in response to conventional weapons.

This could include Ukraine's use of US-supplied ATACMS missiles to hit Russian territory — which Moscow claims happened on Tuesday when six missiles hit the Bryansk region.

The revamped nuclear doctrine generally lowers the bar when it comes to Moscow's use of its atomic arsenal.

So what does this move mean and does it change the likelihood of Russia using its nuclear weapons?

What is Russia's nuclear doctrine?

The first iteration of the document officially titled the "Basic Principles of State Policy on Nuclear Deterrence" was signed by President Putin in 2020.

According to the Kremlin, he approved the latest version which outlines when Russia could dip into its atomic arsenal on Tuesday.

Since Russia invaded Ukraine in 2022, Putin and other Kremlin voices have frequently threatened the West with its nuclear arsenal.

But that hasn't deterred Kyiv's allies from giving it billions of dollars of advanced weapons, some of which have hit Russian soil.

Vladimir Putin's endorsement of the new nuclear policy comes on the eve of the 1,000th day of the war in Ukraine. (AP: Vyacheslav Prokofyev/Sputnik/Kremlin Pool)

The revamped document describes nuclear weapons as "a means of deterrence" and notes their use is an "extreme and compelled measure".

It declares that Russia "takes all necessary efforts to reduce the nuclear threat and prevent aggravation of interstate relations that could trigger military conflicts, including nuclear ones".

"Nuclear deterrence is aimed to ensure that any potential adversary realises the inevitability of retribution in case of an aggression against Russia and its allies," it read.

The new version spells out conditions under which Putin could use a nuclear option in response to a conventional strike.

How likely is a Russian nuclear response?

Not very likely, according to international relations experts.

Australian National University's National Security College associate professor Michael Cohen said Russia was "quite unlikely" to use its nuclear capabilities.

"It's one of those cases where the bark is almost [worse than] the bite," he said.

"Putin has a number of times said, 'look, if you don't be careful, we could use nuclear weapons' … he's never used them, he's just threatening to do so."

Professor Cohen believes this threat from Putin is aiming to get the US to "pull back" its support from Ukraine, but also split the NATO alliance.

He said Russia knows that any nuclear offensive would result in America targeting its nuclear weapons and "even crazier sanctions".

Russian troops have been receiving training on using tactical nuclear weapons such as this Iskander missile. (AP: Russian Defense Ministry Press Service/File)

Aberystwyth University in Wales senior lecturer in international politics Dr Jenny Mathers agrees and says this is a pattern seen with President Putin.

"Putin will warn of the dangers of X or Y … and then when the thing he warns against actually happens, he tends to drop the subject and move onto something else and not really talk about it," she told ABC's RN Breakfast.

"There hasn't been a sort of dramatic escalation that the West has expected."

What does this new nuclear doctrine mean?

The doctrine articulates Moscow could use nuclear weapons "in response to the use of nuclear and other types of weapons of mass destruction" against Russia or its allies.

This includes "in the event of aggression" against Russia, which includes conventional weapons that threaten its sovereignty.

Any aggression against Russia by a non-nuclear power with the "participation or support of a nuclear power" will be seen as their "joint attack" on Russia, the document read.

Dr Mathers said the changes to the doctrine had been in the works for some months before US President Biden relaxed restrictions on Ukraine's use of long-range missiles to hit Russian targets.

Overnight, Moscow claimed six of the missiles had hit a military facility.

Russia has nuclear-capable strategic bombers, including this trio of Tupovlev Tu-95s. (AP: Pavel Golovkin/File)

But Dr Mathers says it's unlikely to trigger a nuclear response.

"It's all part of the messaging that the Putin administration has been doing throughout the war to try and frighten the international supporters of Ukraine," she told ABC's RN Breakfast.

But, Professor Cohen said this move was more about making the nuclear threat "clearer" rather than a tangible change to the doctrine as it was "not the first time" Putin has threatened the use of nuclear weapons.

Where to for Russia from here?

Kremlin spokesman Dmitry Peskov on Tuesday confirmed that a Ukrainian attack on Russia using Western-supplied longer-range missiles could potentially trigger a nuclear response.

Carnegie Russia Eurasia Center's Tatiana Stanovaya noted that Mr Peskov's comment marked the first time the Kremlin explicitly acknowledged "the potential use of nuclear weapons as a response to strikes on Russian territory using long-range missiles".

Professor Cohen said there was going to be more fighting on both sides over the next few months, including cyber attacks, drones, missiles and other conventional warfare tactics.

He noted that any Russian use of nuclear weapons would be partly influenced by how Putin thought US president-elect Donald Trump would respond.

He expects Russia to "hit back as hard as they can" to signal to Trump that the US should walk away from Ukraine.

"Ukrainians will also need to push back to the Russians as hard as they can to signal to Trump … we're about to win, help us," he said.

"You're going to see really intense, vicious fighting by both sides signalling to their external supporters that they're going to prevail."