Senior security official Dmitry Medvedev says Russia is not bluffing when it speaks of the possibility of using tactical nuclear weapons against Ukraine.
Medvedev, who is currently the deputy chairman of Russia's Security Council, also warned Moscow's conflict with the West could escalate into all-out war.
His warning came after the US and Germany became the latest allies to let Ukraine's military use Western-supplied weapons to strike targets inside Russia.
"Russia regards all long-range weapons used by Ukraine as already being directly controlled by servicemen from NATO countries. This is no military assistance, this is participation in a war against us," Medvedev said.
"And such actions could well become a casus belli (an act that provokes a war)."
He said giving Kiev permission by the Western countries to use provided long-range weapons and ammunition to strike targets inside Russia will escalate the Ukraine war to its final stage.
"Nobody today can rule out the conflict's transition to its final stage," Medvedev said.
He said it would be a "fatal mistake" on the part of the West to think that Russia is not ready to use tactical nuclear weapons against Ukraine.
He also spoke of the potential to strike unnamed hostile countries with strategic nuclear weapons. "This is, alas, neither intimidation nor bluffing," said Medvedev.
Kremlin spokesman Dmitry Peskov said several of the US-led NATO countries had "entered a new round of escalating tension and they are doing this deliberately."
Peskov pointed out that the West wanted Kiev forces to continue fighting Russia. "They are in every possible way provoking Ukraine to continue this senseless war."
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London and Paris had already signaled that they were open to easing restrictions on Ukraine striking targets inside Russia.
On Thursday, US President Joe Biden said American-supplied weapons could be used against sites on Russian territory, as well.
Reports said Biden granted Kiev permission to use long-range US missiles to strike inside Russian territory, but only near the border region of Kharkiv, where Russia is making territorial gains.
Following the decision, Germany also gave Ukraine the green light to use German-supplied weapons against Russian targets.
Germany's’ government spokesman Steffen Hebestreit said Berlin had been "jointly convinced" that Kiev had the right to defend itself against Russia.
It came as NATO foreign ministers gathered in the Czech capital of Prague to prepare more arms and ammo for Kiev and decide whether to remove restrictions stopping Kiev from using the weapons, notably long-range missiles, to strike targets on Russian soil.
Ukrainian President Vladimir Zelensky has been pleading its Western backers – the US in particular – for greater NATO engagement in the war while asking permission to use the longer-range weaponry supplied by Washington and its allies to strike targets inside Russia.
Till this week, the Western alliance had been hesitating to allow Kiev to use the Western supplied weapons and munitions on targets inside Russian territory out of fear of reprisal by Moscow.
They feared that such an escalations could drag NATO member states closer to direct conflict with Russia.
Russian President Vladimir Putin says his forces have launched a major operation against the Kharkiv region, which sits across the border from Belgorod, to create a buffer zone to protect Russian frontier villages from Ukraine’s fatal attacks.
Russia launched its "special military operation" in Ukraine in February 2022 to stop what it views as NATO’s eastward expansion and gradual encroachment on the Russian motherland.
On Saturday, Russian missile and drone attack damaged energy facilities in five regions across Ukraine, said the country's national grid operator, Urkenergo.
The air defenses shot down 35 of 53 Russian missiles and 46 of 47 Russian drones, the Ukrainian air force commander said.