Donald Trump says he has ordered two nuclear submarines to be positioned in the"appropriate regions" in a row with former Russian president Dmitry Medvedev.

It comes after Mr Medvedev, who is now deputy chair of Russia 's Security Council, told the US president on Thursday to remember Moscow had Soviet-era nuclear strike capabilities of last resort.

On Friday, Mr Trump wrote on social media:"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. Thank you for your attention to this matter!"

The spat between Mr Trump and Mr Medvedev came after the US president warned Russia on Tuesday it had"10 days from today" to agree to a ceasefire in Ukraine or face tariffs, along with its oil buyers.

Moscow has shown no sign it will agree to Mr Trump's demands.

On Monday, Mr Medvedev accused Mr Trump of engaging in a"game of ultimatums" and reminded him Russia possessed a Soviet-era automated nuclear retaliatory system - or "dead hand" - after Mr Trump told him to "watch his words" and said he's "entering very dangerous territory!"

Mr Medvedev, a close ally of Russian President Vladimir Putin, was referring to a secretive semi-automated Soviet command system designed to launch Russia's missiles if its leadership was taken out in a decapitating strike.

He added:"If some words from the former president of Russia trigger such a nervous reaction from the high-and-mighty president of the United States, then Russia is doing everything right and will continue to proceed along its own path."

He also said "each new ultimatum is a threat and a step towards war" between Russia and the US.

Mr Medvedev served as Russia's president from 2008 and 2012, when Mr Putin was barred from seeking a second consecutive term, but then stepped aside to let him run again.