The threshold for Russia’s use of nuclear weapons was formally lowered by President Vladimir Putin today.

This followed a decision by US President Joe Biden to let Ukraine strike targets inside Russian territory with longer-range missiles supplied by Washington.

The updated doctrine says an attack against Russia by a non-nuclear-armed power with the “participation or support of a nuclear power” will be seen as their “joint attack on the Russian Federation.”

It warns that any massive aerial attack on Russia could trigger a nuclear response but avoids any firm commitment and mentions the “uncertainty of scale, time and place of possible use of nuclear deterrent.”

Russia’s Defence Ministry said Ukraine had fired six US-made ATACMS missiles early today at a military facility in the country’s Bryansk region bordering Ukraine, adding that air defences had shot down five of them and damaged another.

While the doctrine envisages that a conventional strike of that type could trigger a nuclear response, it is formulated broadly to avoid a firm commitment and keep options open.

Asked today whether a Ukrainian attack with longer-range US missiles could trigger a nuclear response, Kremlin spokesman Dmitry Peskov said it could in the case of a conventional strike that poses critical threats to the “sovereignty and territorial integrity” of Russia and its ally Belarus.

Mr Putin first announced the changes to Russia’s nuclear doctrine in September and had previously warned the US and its Nato allies that allowing Ukraine to use Western-supplied longer-range weapons to hit Russian territory would mean that his country and Nato were at war.

Washington’s change of policy came after it said that thousands of North Korean troops had been deployed in the Russian region of Kursk to fight an offensive by Kiev’s forces.