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.

President Donald Trump has ordered the repositioning of two U.S. nuclear submarines. Friday's decision comes after what he calls "highly provocative statements" by former Russian President Dmitry Medvedev.

Rolf Mützenich, former leader of the SPD parliamentary group in Germany's Bundestag, has criticised a "nuclear exchange" between the United States and Russia, calling it irresponsible. Source: German newspaper Der Spiegel,