The decision to change Russia's official nuclear doctrine is the Kremlin's answer to a reported decision by the administration of U.S. President Joe Biden to allow Ukraine to fire American long-range missiles deep into Russia.

Vladimir Putin signalled on Tuesday he was ready to start a nuclear war after Ukraine struck Russia with Western missiles for the first time.

A decree signed by the Russian leader, though long planned, came days after President Biden authorized the use of U.S.-supplied missiles by Ukraine for strikes inside Russia.