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.

In a post on social media, 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 posted several comments in recent days threatening the US in response to Trump's ultimatum to Moscow to agree to a ceasefire in Ukraine, or face tough sanctions.

In a post on Truth Social, Trump 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".

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

In a post on X, the former Russian president said that "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.