A key ally of Vladimir Putin has warned that "nuclear apocalypse" is more likely if the West continues to supply Ukraine with powerful weapons.
The former Russian president Dmitry Medvedev made the chilling threat after US president Joe Biden said he will support a joint effort to train Ukrainian pilots to fly F-16 fighter jets.
Russia, which has the largest nuclear arsenal on the planet, has repeatedly threatened the West with an escalation of the conflict if they back Kyiv.
In his latest rant against Moscow's enemies, Medvedev said: "The more weapons are supplied, the more dangerous the world will be.
"And the more destructive these weapons are, the more likely the scenario becomes of what is commonly called a nuclear apocalypse."
The West is determined to help Ukraine beat Russia but is wary of triggering a direct confrontation between the NATO military alliance and Moscow but Medvedev suggested that NATO did not appear to be taking the threat of nuclear war seriously.
The deputy chair of Russia's Security Council said: "They are wrong. And at some point, events can move in a completely unpredictable scenario. And the responsibility will lie entirely with the North Atlantic Alliance."