Russia has signalled it will retaliate with full-scale nuclear war after Ukraine launched US-made missiles over the border on Tuesday. 

Vladimir Putin's foreign minister Sergei Lavrov chillingly warned that Moscow would 'react accordingly' as he described the strike as a signal Kyiv 'wanted to escalate'. 

The terrifying sabre rattling was confirmed by the Russian President's close ally Dmitry Medvedev. Posting on X, the Deputy Chair of Russia's security council said it meant 'World War III'.

In a dramatic escalation on the 1,000th day of the conflict, Kyiv launched six 'ATACM' missiles from an undisclosed location over the border into Russian territory. 

The strikes triggered a fiery explosion at a depot in Karachev, believed to be storing ammunition supplied by North Korea, around 75 miles from the Ukrainian border. 

At the same time, Putin provocatively signed off changes to the country's laws on nuclear weapons to make it easier for them to be deployed against Ukraine in retaliation.

The amendments allow Russia to launch a nuclear attack in response to a conventional weapons strike – such as one by long-range missiles.

It comes as Sir Keir Starmer offered his biggest hint yet that No.10 would authorise Ukraine to fire UK Storm Shadow missiles into Russia.

He said the G20 summit in Brazil: 'I've been doubling down in my clear message that we need to ensure Ukraine has what is needed for as long as needed to win this war against Putin.

'I am not going to go into operational details. You would not expect me to. The only winner in that would be Putin and it would undermine Ukraine.' 

He added that the rhetoric currently coming from Russia was 'irresponsible' and would not deter the UK's reosponse.  

 Zelenksy's plea to use UK Storm Shadow missiles on targets in Russia has so far been rebuffed by Downing Street for fears of escalating the conflict. 

A senior defence source said the UK prefers to 'confirm by use', for example, when a missile strikes.

Meanwhile, there is growing concern about Europe's ability to respond to Russia's increased use of hybrid warfare and sabotage attacks. 

A former defence official, has warned that it is 'totally unprepared' for the Kremlin's escalating response, The Guardian has reported. 

The senior European defence official, who recently left their position, said the block lacked the resources to counter Russian sabotage attacks. 

Such attacks are expected to increase in frequency after Biden approved the use of ATACMS earlier this week. 

Earlier this year, US and German intelligence services reportedly uncovered a Russian plot to assassinate several defense industry executives across Europe.

US security sources have indicated that the decision to allow ATACMS strikes was a direct response to Russia recruiting thousands of North Korean troops.

Around 10,000 soldiers provided by Pyongyang are expected to take part in a Russian bid to liberate the province of Kursk, parts of which have been seized by Ukrainian forces.

Yesterday, Mr Zelensky raised the prospect that the number of North Korean troops could even swell to 100,000.

Neither the US nor Ukraine has confirmed the use of the missiles, with Zelenksy merely saying his country has 'long-range capabilities'. 

But Russian news agencies explicitly referred to ATACMs in their state-supported reports.

Following the attack, Moscow's emergency ministry announced it had  begun mass-producing mobile bomb shelters that can protect against shockwaves and radiation from a nuclear blast.

The new 'KUB-M' shelters will offer protection for 54 people for up to two days against natural and man-made hazards, Moscow's emergency ministry has said.

'The mobile shelter is a multifunctional structure that provides protection for people from various threats, including natural disasters and man-made accidents,' its research institute explained, calling it 'an important step towards improving the safety of citizens'. 

 They can easily be transported on a truck and connected to water supplies, and can also be deployed in Russia's vast northern permafrost, it added. 

In anticipation of Ukraine receiving permission to use ATACMS, Russia's air force is said to have redeployed many of its fighter jets and strategic bomber aircraft away from the airbases in range of the missiles to sites further east.

But there are still more than 200 military facilities that could be battered by the US-manufactured munitions from positions along the 800-mile-long frontline.

Among the first likely targets for Ukrainian strikes could be the Kuzminka Military Base, a critical staging area for Putin's forces between the Russian city of Rostov-on-Don and the Ukrainian border which acts as a hub for vehicle storage, troop assembly and operational planning.

Russia's 381st artillery regiment is also located close to the Kuzminka base along with several logistics centres.

Meanwhile, in Kursk, Kyiv's troops could seek to target the headquarters of Russia's 448th Missile Brigade or regional assembly points used to gather troops preparing to push back the Ukrainian incursion in the region.

Targeting these sites could cripple Russian logistics, command, and combat support, significantly reducing Moscow's offensive capabilities in occupied Ukrainian territory​ and harming efforts to retake territory in Kursk.

However, the overall supply of ATACMS missiles to Ukraine is short, so US officials and analysts have questioned whether allowing Ukraine to use the weapons systems is really worth it given the potential consequences that could ensue.

Jennifer Kavanagh, director of military analysis at the Defense Priorities think tank, was critical of the Biden administration's decision to approve the use of ATACMS on targets in Russia.

'Expanding Ukraine's ability to launch offensive strikes with Western weapons inside Russia will not alter the trajectory of the war or help Kyiv gain an advantage against a better equipped and more resilient adversary.

'Any escalation could reverberate on Ukraine itself. With the Biden administration on its way out and the incoming Trump administration indicating an intention to end the war, Putin has little incentive to act with restraint in his retaliation toward Kyiv.'

But proponents of the policy say that even a few strikes deeper inside Russia would force its military to change deployments and expend more of its resources.

George Barros, leader of the Russia team and GEOINT team at ISW that compiled the list of targets, sought to highlight the way in which ATACMS could impact Putin's troops and campaigned for the Biden administration to allow strikes beyond Kursk as a result.

'Reminder that there are hundreds of valid, legal, legitimate, and operationally consequential military targets in range of Ukrainian ATACMS,' he wrote.

'The Biden Administration's shift to allow ATACMS use in Russia is a good thing.'

Matthew Savill, the director of military sciences at the Royal United Services Institute, said of Ukraine's ATACMS attacks: 'Many of the best targets – helicopters and glide-bomb armed aircraft attacking Ukrainian cities and troops in the north or in Kursk have largely moved to airbases outside the range of ATACMS.

'This still leaves plenty of opportunities to strike military headquarters and ammunition or supply locations supporting Russian and North Korean troops, but this would be a reduced impact from when the Ukrainians first requested these weapons.'

It remains to be seen whether Britain and France will follow America's lead by allowing Kyiv's troops to attack Russian targets with Storm Shadow and SCALP long-range missiles - though it is expected the restrictions will soon be relaxed.