"Potentially, this is extremely dangerous, it will mean bringing the conflict to a whole new level, which, of course, will not bode well from the point of view of global and pan-European security," Kremlin spokesman Dmitry Peskov told reporters.

The warning comes on the eve of a key donor meeting as Western countries consider sending more powerful weapons to Ukraine.

On Friday, the United States will gather its allies at its airbase in Ramstein for a new round of talks on backing Ukraine militarily.

Peskov spoke after Moscow's ambassador to the United States Anatoly Antonov said Russia would retaliate if Ukraine uses Western-supplied weapons to target Russia or the Crimea peninsula, annexed from Ukraine in 2014.

"It should become obvious to everyone: no matter what weapons the Americans or NATO supply to the Zelensky regime, we will destroy it," he said, referring to Ukrainian President Volodymyr Zelensky.

"It is simply impossible to defeat Russia," he said in comments released by the embassy.

He said that US rhetoric over Ukraine was becoming "more and more belligerent."

By insisting that Crimea was part of Ukraine and saying that Kyiv can use US weapons to protect its territory Washington "is essentially pushing the Kyiv regime to commit terror acts in Russia," Antonov said.

Separately, former president Dmitry Medvedev warned that the West's continued support for Ukraine could lead to nuclear war.

"A nuclear power losing in a conventional war can provoke the outbreak of a nuclear war," he wrote.

"Nuclear powers have not lost major conflicts on which their fate depends."

Peskov said Medvedev's comments were in line with Russia's nuclear doctrine.