Former Russian President Dmitry Medvedev has said the supply of more advanced US weaponry to Ukraine will only trigger more retaliatory strikes from Russia, up to the extent of Russia's nuclear doctrine.
"All of Ukraine that remains under Kyiv's rule will burn," journalist Nadana Fridrikhson quoted him as saying in a written interview with her.
Fridrikhson asked Medvedev, who as deputy chairman of the Security Council has become one of Russia's most hawkish pro-war figures since its invasion of Ukraine, whether the use of longer-range weapons might force Russia to negotiate with Kyiv, reports Reuters.
"The result will be just the opposite," Medvedev replied, in comments that Fridrikhson posted on her Telegram channel.
"Only moral freaks, of which there are enough both in the White House and in the Capitol, can argue like that."
The Pentagon said on Friday that a new rocket that would double Ukraine's strike range was included in a $2.175 billion US military aid package.
With the first anniversary of the invasion approaching on Feb 24, Russian forces have been on the back foot for the last eight months, and do not fully control any of the four Ukrainian provinces that Moscow has unilaterally declared part of Russia.
President Vladimir Putin casts Russia's campaign in Ukraine as an existential defence against an aggressive West and has, like Medvedev, several times brandished the threat of a nuclear response, saying Russia will use all available means to protect itself and its people.
Asked what would happen if the weapons that Washington has promised Ukraine were to strike Crimea - which Russia seized from Ukraine in 2014 - or deep into Russia, Medvedev said Putin had addressed the matter clearly.
"We don't set ourselves any limits and, depending on the nature of the threats, we're ready to use all types of weapons. In accordance with our doctrinal documents, including the Fundamentals of Nuclear Deterrence," he said. "I can assure you that the answer will be quick, tough and convincing."
Russia's nuclear doctrine allows for a nuclear strike after "aggression against the Russian Federation with conventional weapons when the very existence of the state is threatened".