US President Donald Trump says he has ordered two nuclear submarines to "be positioned in the appropriate regions" in response to "highly provocative" comments by former Russian President Dmitry Medvedev.

Trump said he acted "just in case these foolish and inflammatory statements are more than just that. Words are very important, and can often lead to unintended consequences, I hope this will not be one of those instances".

He did not say where the two submarines were being deployed.

Medvedev has recently threatened the US in response to Trump's ultimatum to Moscow to agree to a ceasefire in Ukraine, or face tough sanctions.

Russia and the US possess the most nuclear arms in the world.

But Trump did not say whether he was referring to nuclear-powered or nuclear-armed submarines in his post on Truth Social.

In the post, the US president wrote: "Based on the highly provocative statements of the former president of Russia, Dmitry Medvedev, who is now the deputy chairman of the Security Council of the Russian Federation, I have ordered two nuclear submarines to be positioned in the appropriate regions".

Trump and Medvedev have recently been involved in a series of personal attacks against each other on social media.

On Monday, Medvedev accused Trump of playing "the ultimatum game with Russia".

In a post on X, he said "each new ultimatum is a threat and a step towards war".

Earlier this month, Medvedev described Trump's ultimatum as "theatrical", saying that "Russia didn't care".

And writing on Telegram on Thursday, Medvedev warned of a "dead hand" threat - which some military analysts understood as a reference to the codename of Russia's retaliatory nuclear strikes control system.

On Thursday, Trump described Medvedev as "the failed former president of Russia, who thinks he's still president".

Trump also warned Medvedev to "watch his words", adding that "he's entering very dangerous territory!"

The White House told the BBC they had no further comment on Trump's remarks.

The Kremlin has not publicly commented on the latest statement by Trump.