A senior Russian figure and close adviser to Vladimir Putin has warned of a "full-fledged nuclear war" if Nato continues "pumping weapons into Ukraine".
Dmitry Medvedev, deputy chairman of Russia's security council, made the threat shortly after Finland moved another step closer to applying to join the Western alliance.
The former president of Russia said in a Telegram post: "Nato countries pumping weapons into Ukraine, training troops to use Western equipment, sending in mercenaries and the exercises of Alliance countries near our borders increase the likelihood of a direct and open conflict between Nato and Russia."
"Such a conflict always has the risk of turning into a full-fledged nuclear war," Medvedev said before adding: "This will be a disastrous scenario for everyone."
Nato countries have been arming Ukraine since the invasion, pouring resources into the nation to help it repel Russian forces.
Included in the weaponry Nato nations have so far supplied are next generation light anti-tank weapons (NLAWs) — shoulder-mounted guided missiles which have proven to be useful against tanks.
More than 700 Switchblade drones — small enough to be carried in a backpack — and 121 Ghost drones have also been provided.
The UK has been among the countries to provide weaponry as part of the defence of Ukraine.
In April, Boris Johnson visited Kyiv in person, promising an additional 120 armoured vehicles, in addition to anti-ship missile systems to support Ukraine in the Black Sea.
Russia's invasion in February has sparked a reaction from Western countries and is set to see geopolitical alliances redrawn.
Most notably, it seems increasingly likely that Finland and Sweden could join Nato in the coming months.
On Thursday, Finland's president and prime minister, Sauli Niinistö and Sanna Marin, said their nation must apply to join Nato "without delay".
It marks a major change in the Scandinavian countries' foreign policy, and drops a decades-old agreement of military neutrality.
Niinistö and Marin made the call in a joint statement, and said: “We hope that the national steps still needed to make this decision will be taken rapidly within the next few days.”
Asked whether Finland’s accession posed a direct threat to Russia, Kremlin spokesman Dmitry Peskov said: “Definitely. NATO expansion does not make our continent more stable and secure.
“This cannot fail to arouse our regret, and is a reason for corresponding symmetrical responses on our side,” Peskov added, without elaborating.
Russian officials have spoken in the past about potential measures including stationing nuclear-armed missiles on the Baltic Sea.
Watch: Finland poised for Nato membership as Ukraine war crimps Russian gas
Last month, research by Demoskop and commissioned by the Aftonbladet newspaper showed 57% of Swedes now favoured Nato membership, up from 51% in March.
The March poll was the first to show a majority of Swedes in favour of joining the alliance. In January, a month before the invasion, just 37% were in favour of Nato membership.
Sweden is currently in the midst of a review into its security policy, which will conclude at the end of May.
It is expected to announced its decision on whether or not to join Nato this weekend.
It has been expected that Putin would used the annual Victory Day parade on 9 May.
But experts said his speech could be interpreted as an "encouraging" sign he is stepping back from the catastrophic consequences of launching nuclear weapons.
Western officials had warned prior to the speech that the Russian president could use the opportunity to instigate a mass mobilisation of Russian troops; declare a global war on Nazis; or even raise the threat of using nuclear weapons.
Former UK ambassador to Russia, Sir Tony Brenton, told Sky News that Putin had recently shown "disturbing signs" of "losing it", in his speeches surrounding the war, but the speech today was "coherent" and a "very professional performance".
He added the speech had "pushed all the right patriotic buttons in the Russian political atmosphere".
Sir Tony added: "It is rather encouraging that it leaves me with the feeling that we are dealing with a rational individual there with whom hopefully in time it will be possible to do a sensible deal to bring this whole mess to an end."