Donald Trump says US nuclear submarines are being repositioned in response to "provocative statements" from Russia's former president, Dmitry Medvedev.
In a Truth Social post on Friday, local time, 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, 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 later told reporters at the White House he had ordered the submarines be moved "on the basis of safety for our people".
"A threat was made by a former president of Russia, and we're going to protect our people," he said.
Mr Medvedev, who is close to President Vladimir Putin, has recently been mocking Mr Trump on social media.
After Mr Trump said he was giving Russia a 10-day deadline to agree to a peace deal in Ukraine earlier this week, Mr Medvedev accused Mr Trump of taking a "step towards war".
In an X post on Monday, he wrote:
"Trump's playing the ultimatum game with Russia: 50 days or 10… He should remember 2 things:
1. Russia isn't Israel or even Iran.
2. Each new ultimatum is a threat and a step towards war. Not between Russia and Ukraine, but with his own country. Don't go down the Sleepy Joe road!"
Mr Medvedev is seen as a hardline, anti-West attack dog for Mr Putin. He frequently makes inflammatory statements about the US and Ukraine, sometimes featuring implicit or explicit threats of nuclear escalation.
On Thursday, Mr Trump used social media to tell Mr Medvedev he was entering "very dangerous territory" and he should "watch his words".
In recent weeks, Mr Trump has expressed increasing frustration with his efforts to broker a peace deal between Russia and Ukraine.
Several weeks ago, he said he was giving Russia a 50-day deadline to agree to a deal. On Monday, while visiting Scotland, he said he was shortening that deadline. On his flight back to the US on Tuesday, he said the deadline was "10 days from today".
He has threatened economic penalties, including secondary sanctions on Russia's trading partners, if it does not comply.
In the days since, Russia has continued its attacks on Ukraine. On Thursday, it launched its worst air strike on Kyiv for the year, firing more than 300 drones and eight missiles into the capital. At least 31 people were killed and more than 150 wounded.
Neither the Pentagon nor the White House have provided further details about Mr Trump's comments regarding the submarine movements.
The US Navy's nuclear submarines are nuclear-powered, but not all are nuclear-armed.
Mr Trump last month said more American weapons, including Patriot missile systems, would soon be provided to Ukraine via other NATO nations.
On Friday, Germany said it would deliver the launchers to Ukraine in the coming days.
It said additional system components would be provided in the next two to three months to strengthen Ukraine's air defence with more Patriot batteries.
