As the G20 came to an end, Mr Putin, who is under an international arrest warrant, missed the summit but upped his threat to use nuclear warheads after Ukraine, with the permission of outgoing American President Joe Biden, fired US-supplied missiles deep into Russia.

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

President Vladimir Putin has 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.