President Vladimir Putin has updated Russia’s nuclear doctrine, two days after his US counterpart Joe Biden granted Ukraine permission to strike targets deep inside Russia with American-made weapons.

Russian President Vladimir Putin on Tuesday signed a revised nuclear doctrine declaring that a conventional attack on Russia by any nation that is supported by a nuclear power will be considered a joint attack on his country.

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.