Vladimir Putin has signed a decree lowering the threshold for Russia's use of nuclear weapons.  It follows a US decision to allow Ukraine to carry out long-range missile strikes on Russia using American weapons.  Ukraine has carried out its first such attack, with Russia confirming that US ATACMS missiles have hit its Bryansk region.

Russia’s revised nuclear doctrine states that a conventional attack on Russia by any nation that is supported by a nuclear power will be considered a joint attack.  An attack on Russia with conventional missiles, drones or aircraft could meet the criteria for a nuclear response.

The Kremlin urged other countries to study the changes.  Kremlin spokesperson Dmitry Peskov described the decree as “a very important text” and said "it should become a subject to a very deep analysis".

Recently Russia warned it would deliver “an appropriate and tangible” response to the US President's decision to let Ukraine use ATACMS missiles to strike the country.  Russia's foreign ministry said that such an attack "would represent the direct involvement of the United States and its satellites in hostilities against Russia”.

Joe Biden’s change of policy comes two months before Donald Trump is sworn in as the new US President.  Mr Trump has claimed that he will help bring a swift end to the war in Ukraine.

Reeta Chakrabarti presents BBC News at Ten reporting by Steve Rosenberg, Nick Eardley, Vicki Young and Lyse Doucet.