Senior Russian security official Dmitry Medvedev said on Friday that Russia was not bluffing when it spoke of the possibility of using tactical nuclear weapons against Ukraine and warned Moscow's conflict with the West could escalate into all-out war.

Medvedev, deputy chairman of Russia's Security Council and a former Russian president, said Moscow's conflict with the West was developing according to the worst case scenario and that "nobody today can rule out the conflict's transition to its final stage."

"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)."

Medvedev, who has become one of the Kremlin's most hardline hawks, was commenting after four U.S. officials told Reuters on Thursday that U.S. President Joe Biden had quietly authorised Kyiv to launch U.S.-supplied weapons at military targets inside Russia that are supporting an offensive against the northeastern Ukrainian city of Kharkiv.

Medvedev, whose statements diplomats say give a flavour of what senior people in the Kremlin are thinking, said it would be a "fatal mistake" on the part of the West to think that Russia was 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.

"The current military conflict with the West is developing according to the worst possible scenario. There is a constant escalation when it comes to the firepower of NATO weapons being used. Therefore, nobody today can rule out the conflict's transition to its final stage," he said.