By Guy Faulconbridge MOSCOW (Reuters) -Russian President Vladimir Putin issued a warning to the United States on Tuesday, lowering the threshold for a nuclear strike just days after the administration of Joe Biden reportedly allowed Ukraine to fire American missiles deep into Russia.

He declared that a conventional attack by any nation supported by a nuclear power will be considered a joint attack on Russia.

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.