Defence minister Antti Häkkänen Finland and Europe’s security environment had “fundamentally and significantly changed” since Russia’s full-scale invasion of Ukraine began in 2022.

The Nordic nation abandoned decades of military neutrality to join Nato in 2023 over mounting concerns about the threat posed by Russia.

Under Finland’s 1987 Nuclear Energy Act, the import, manufacture, possession and detonation of nuclear explosives is prohibited on Finnish soil – even during wartime.

The government proposal would change that, making it possible to “bring a nuclear weapon into Finland, or to transport, deliver or possess one in Finland, if it is connected to the military defence of Finland”, Häkkänen said.

“The amendment is necessary to enable Finland’s military defense as part of the alliance and to take full advantage of Nato’s deterrence and collective defence,” he told a news conference on Thursday.

Nato’s founding principle of collective defence – that an attack on one member nation would be treated as an attack on all – underpins the alliance’s nuclear deterrence stategy.

As it contains several nuclear powers, this means a direct attack on a member brings with it the risk of a nuclear response. US nuclear weapons are stationed in several European nations, according to the Center for Arms Control and Non-proliferation.

The Finnish government’s proposal requires changes to both the country’s Nuclear Energy Act and the criminal code.

Its governing right-wing coalition, which holds a majority in parliament, said the proposal had been circulated for consultation until 2 April before it is formally laid out.

Finland shares a 1,340km (832-mile) border with Russia – the longest of any EU or Nato member state – and its leaders have repeatedly warned that the country’s security environment had deteriorated since Russia’s invasion of Ukraine.

The Nordic nation became the 31st member of Nato in April 2023, widely seen as a strategic setback for Russian President Vladimir Putin, who had long criticised the military alliance’s eastward expansion. Its neighbour Sweden joined Nato in 2024.

Since then, Nato has increased its military presence in the Arctic and Baltic Sea, as well as along the newly expanded eastern flank.

The proposal comes as European countries step up defence co-operation in response to the war in Ukraine and broader global instability.

Several Nato states suffered air traffic disruption after drones were sighted over airports and airbases last year, which some European officials attributed to “hybrid warfare” on the part of Russia.

Moscow denied involvement, but the incidents prompted renewed discussions about collective defence.

On Monday, France and Germany announced plans to deepen co-operation with European partners on nuclear deterrence.

Swedish Prime Minister Ulf Kristersson said last week that the country’s doctrine of not stationing foreign troops or nuclear weapons on its territory “would not apply” if Sweden found itself “in a completely different situation”.

Voislav Torden, 38, a senior member of the Russian far-right mercenary group Rusich, was found guilty of four charges by a court in the Finnish capital, Helsinki, on Friday, while he was acquitted of a fifth charge.

The charges relate to an ambush and firefight that occurred in the Luhansk region of Ukraine, which killed 22 Ukrainian soldiers and injured four others. Torden denies the allegations.

It marks the first time charges have been brought and heard in a Finnish court over allegations of war crimes in Ukraine.

Torden, previously known as Yan Petrovsky, was a founding member of Rusich, which operated in the eastern Donbas region as part of pro-Russian separatist fighting against Ukraine. Rusich is a subunit of the Wagner group.

It was alleged that, on 5 September 2014, Torden led his men as part of an ambush of Ukrainian soldiers by pretending to be Ukrainian, before setting fire to a truck and car belonging to the unit.

Twenty-one Ukrainian troops were killed and a further five injured, the indictment said.

The court in Helsinki found there was insufficient evidence to conclude that Rusich was specifically responsible for the ambush, as there were several other groups involved.

However, it found Torden guilty on all other counts, including that he was in charge of the Rusich mercenaries present during the ambush, who killed at least one Ukrainian soldier and injured another.

His men were also found to have mutilated a wounded soldier by “making the Rusich group symbol on his face”.

Torden was found to have distributed “degrading” images of the soldier and to have posted on social media that Rusich would “not grant mercy”.

A panel of three judges unanimously found him guilty of the latter four charges, writing that the most serious – of killing a soldier – was “comparable to murder due to its brutality and cruelty”.

While the court held that there was insufficient evidence to find him culpable for the deaths of the 21 other Ukrainian soldiers, it ordered him to pay compensation to the family of the soldier whose death he was found responsible for.

Torden has consistently denied the allegations levelled against him, Finland’s public broadcaster reports. He intends to appeal against the conviction, according to national newspaper Ilta-Sanomat.

Torden’s lawyer, Heikki Lampela, said the verdict had taken them by surprise.

“There was no evidence that he killed the wounded or gave the order to do so,” he told Finland’s public broadcaster, Yle, adding that Torden was “equally confused” he had received a life sentence “for an act he did not commit”.

Torden was arrested at Helsinki-Vantaa Airport in July 2023 at the request of the Ukrainian government, which sought to extradite him.

That request was rejected by Finland’s Supreme Court over concerns he would not receive a fair trial in Ukraine – but he was still able to be tried in Helsinki as he was accused of crimes under international law.

Yle reports that similar charges have been tried domestically relating to acts in countries including Rwanda and Iraq.

The office of Ukraine’s prosecutor general hailed the court’s ruling as a “key milestone” in holding perpetrators of “grave violations of international humanitarian law accountable”.

It added in a statement that Ukrainian officials had ensured that the court had heard from victims and witnesses in Ukraine during the trial, adding that it would continue to work with partners internationally to “ensure there is no impunity for war criminals”.

Nato chief Mark Rutte said the mission, dubbed “Baltic Sentry”, would involve more patrol aircraft, warships and drones.

His announcement was made at a summit in Helsinki attended by all Nato countries perched on the Baltic Sea – Finland, Estonia, Denmark, Germany, Latvia, Lithuania, Poland and Sweden.

While Russia was not directly singled out as a culprit in the cable damage, Rutte said Nato would step up its monitoring of Moscow’s “shadow fleet” – ships without clear ownership that are used to carry embargoed oil products.

Tensions between Nato countries and Russia have been mounting relentlessly since Russia’s full-scale invasion of Ukraine in February 2022.

“There is reason for grave concern” over infrastructure damage, Rutte said. He added that Nato would respond to such accidents robustly, with more boarding of suspect vessels and, if necessary, their seizure.

He declined to share more details on the number of assets that will take part in the Baltic Sentry initiative, as he said this could change regularly and that he did not wish to make “the enemy any wiser than he or she is already”.

Undersea infrastructure is essential not only for electricity supply but also because more than 95% of internet traffic is secured via undersea cables, Rutte said, adding that “1.3 million kilometres (800,000 miles) of cables guarantee an estimated 10 trillion-dollar worth of financial transactions every day”.

In a post on X, he said Nato would do “what it takes to ensure the safety and security of our critical infrastructure and all that we hold dear”.

There has been an uptick in unexplained damage to undersea infrastructure in the Baltic in recent months.

The most recent accident to undersea infrastructure saw an electricity cable running between Finland and Estonia be cut in late December.

Finnish coast guard crew boarded the oil tanker Eagle S – which was sailing under a Cook Islands flag – and steered it into Finnish waters, while Estonia deployed a patrol ship to protect its undersea power cable.

On Monday, Risto Lohi of Finland’s National Bureau of Investigation told Reuters that the Eagle S was threatening to cut a second power cable and a gas pipe between Finland and Estonia at the time it was seized.

Estonia’s Foreign Minister Margus Tsahkna said in December that damage to submarine infrastructure had become “so frequent” that it cast doubt on the idea the damage could be considered “accidental” or “merely poor seamanship”.

Tsahkna did not accuse Russia directly. Neither did Swedish Prime Minister Ulf Kristersson, who on Sunday said that while Sweden was not jumping to conclusions or “accusing anyone of sabotage without very strong reasons”, it was also “not naive”.

“The security situation and the fact that strange things happen time and time again in the Baltic Sea also lead us to believe that hostile intent cannot be ruled out.”

“There is little evidence that a ship would accidentally and without noticing it… without understanding that it could cause damage,” he said.

The authorities said on Thursday that they believe the anchor of the Eagle S, a tanker registered with the Cook Islands, may have damaged the Estlink 2 cable, which became disconnected on Wednesday.

The vessel is thought to be part of Russia’s “shadow fleet”, which is made up of ships that carry embargoed Russian oil products.

It is the latest in a series of incidents in recent years, in which underwater cables in the Baltic region have been either damaged or severed completely.

Fingrid, the operator of Finland’s national grid, said Estlink 2 remained out of service but that the damage “did not endanger the operation of the electricity system” in the country.

Repairs are expected to take “several months”.

“From our side we are investigating grave sabotage,” said Robin Lardot, director of Finland’s National Bureau of Investigation (NBI).

President Alexander Stubb posted on X that he had been given an “overview” of the cable breach from authorities.

He underlined the need to “avert the risks” posed by vessels that are part of the shadow fleet.Finnish police have said the case is being investigated as “aggravated criminal mischief”.

The damaged cable had a transmission capacity of 650 megawatts and is 170km long (105 miles), 145km of which is submerged. The fault was located on Thursday in the submarine section.

The EU has threatened to impose further sanctions against Russia as a result of the incident and said it was “strengthening efforts to protect undersea cables”.

“We strongly condemn any deliberate destruction of Europe’s critical infrastructure,” the European Commission and the EU’s foreign policy chief, Kaja Kallas, said in a joint statement.

Estonia’s Foreign Minister Margus Tsahkna said on Thursday that damage to critical submarine infrastructure has become “so frequent” that it casts doubt on the idea this damage could be considered “accidental” or “merely poor seamanship”.

“We must understand that damage to submarine infrastructure has become more systematic and thus must be regarded as attacks against our vital structures,” Tsahkna said in a press release.The foreign ministry added that Eagle S has been escorted to a Finnish port.

“In addition to circumventing sanctions, the shadow fleet is a security threat in the Baltic Sea and we cannot just sit and watch,” Tsahkna continued.

A telecommunications cable running between Finland and Germany was severed in November, and an internet link between Lithuania and Sweden’s Gotland Island stopped working at around the same time.

German prosecutors are still investigating the explosion of Nord Stream gas pipelines between Russia and Germany in 2022.

And in October 2023 a natural gas pipeline between Finland and Estonia was severely damaged.

Finnish officials later said the incident had been caused by a Chinese container ship dragging its anchor.