US President Donald Trump on Friday said he had ordered two nuclear submarines to be positioned in regions near Russia in response to threats from former Russian president Dmitry Medvedev.

"Based on the highly provocative statements of the Former President of Russia, Dmitry Medvedev... I have ordered two Nuclear Submarines to be positioned in the appropriate regions, just in case these foolish and inflammatory statements are more than just that," Trump posted on his Truth Social platform.

"Words are very important, and can often lead to unintended consequences, I hope this will not be one of those instances."

Trump did not say whether he meant nuclear-powered or nuclear-armed submarines. He also did not elaborate on the locations, which are kept secret by the US military.

The United States and Russia control the vast majority of the world's nuclear weaponry, and Washington keeps nuclear-armed submarines on patrol as part of its so-called nuclear triad of land, sea and air-launched weapons.

Trump did not refer specifically to what Medvedev had said to prompt the highly unusual public display of nuclear saber-rattling.

Medvedev: 'playing the ultimatum game'

Medvedev on Monday harshly criticised Trump's threat of new sanctions against Russia over the continuing invasion of Ukraine.

Accusing Trump of"playing the ultimatum game," he posted on X that Trump "should remember" that Russia was a formidable force.

"Each new ultimatum is a threat and a step towards war. Not between Russia and Ukraine, but with his own country," the Russian official said.

Medvedev is currently deputy chairman of Russia's Security Council and a vocal proponent of President Vladimir Putin's war in Ukraine – and generally antagonistic relations with the West.

He served one term as president between 2008-2012, effectively acting as a placeholder for Putin, who was able to circumvent constitutional term limits and remain in de facto power.