Former Russian president Dimitri Medvedev has threatened the United States with nuclear war via its 'fabled Dead Hand' nuclear deterrent after Donald Trump told Putin's right-hand man to "watch his words".

Posting to his Truth social media platform, the US President declared he did not "care what India does with Russia," after making a trade deal with the former's nuclear rival, Pakistan, before taking aim at Medvedev.

Trump warned: "Tell Medvedev, the failed former President of Russia, who thinks he's still President, to watch his words. He's entering very dangerous territory!" It comes after a chilling map pinpointed the 10 safest nations to flee to during a nuclear World War 3.