The Kremlin has tried to calm tensions in a feud between Trump and Medvedev that spiraled into nuclear threats.

Trump also rebuked Medvedev in July, accusing him of throwing around the "N (nuclear) word" after the Russian official criticised U.S. strikes on Iran and said "a number of countries" were ready to supply Iran with nuclear warheads.

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,