Vladimir Putin's chief spokesman has chillingly warned the surprise change to Russia's nuclear weapons doctrine was intended to send a "signal" to the West.

On Wednesday, Putin warned Russia could respond to conventional missile strikes with nuclear weapons, effectively lowering the threshold for a strike and putting the world on a path toward nuclear war.

The Russian leader also indicated that Moscow would treat any attack backed by a nuclear-armed country as a coordinated assault.

That means if the Kremlin deems a Ukrainian assault on Russian soil using US, French, or British missiles to represent a "critical threat" to Russia’s sovereignty, Moscow will consider Kyiv’s Western allies as the attackers too.

On Thursday, Kremlin spokesperson Dmitry Peskov confirmed updates had been made to Russia's nuclear doctrine, in a document titled 'The Foundations of State Policy in the Sphere of Nuclear Deterrence'.

When asked if these changes were intended as a message to the West, Peskov said: "This should be considered a definite signal," according to the Kyiv Independent.

This comes amid discussions in the US and the UK about whether to allow Ukraine to launch conventional Western missiles deep into Russian territory.

Mr Peskov explained the updates to the country's nuclear weapon rules serve as "a signal that warns these countries about the consequences if they participate in an attack on our country by various means, and not necessarily nuclear ones".

He added the world is witnessing an "unprecedented confrontation" in Ukraine provoked by the "direct involvement of Western countries, including nuclear powers".

Russia has the largest nuclear arsenal globally, with a stockpile of 6000 warheads.

US Secretary of State Antony Blinken slammed President Putin's nuclear announcement, calling it "irresponsible".

He told the BBC: "I think a lot of people in the world were clear about that when he rattled off nuclear weapons earlier, including China."

Outgoing NATO Secretary General Jens Stoltenberg echoed this: "Russia's nuclear rhetoric is dangerous and reckless. We are closely watching what Russia is doing."

Keir Giles, a senior consulting fellow at the London-based Chatham House think tank, said that the change in nuclear doctrine showed that Russia was worried. He told Al Jazeera: “Whenever Russia detects that there is a risk for the development of the support for Ukraine that it would dislike, the nuclear threats ramp up."