Former Russian president Dmitry Medvedev has become embroiled in a tense back-and-forth on social media that prompted U.S. President Donald Trump to announce he had ordered the re-positioning of two U.

The United States is moving two of its nuclear submarines to regions near Russia, US President Donald Trump said, following doomsday threats over the war in Ukraine from former Russian President 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 said. 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.