In a thinly veiled warning to the West over its support of Ukraine, the Kremlin leader also said he would consider any assault that was supported by a nuclear power to be a joint attack on Moscow.

He was speaking at a meeting of Russia's security council, where he proposed changes to the country's nuclear doctrine - the legal framework which outlines the conditions for the use of nuclear weapons.

The current document says Russia can only strike first if the security of the state is at risk.

Moscow has repeatedly signalled in recent months that it was mulling an expansion of the doctrine, but this is the first time any details have been provided.

Highlighting one change in particular, Mr Putin said: "It is proposed that aggression against Russia by any non-nuclear state, but with the participation or support of a nuclear state, be considered as their joint attack on the Russian Federation."

"We will consider such a possibility when we receive reliable information about a massive launch of air and space attack assets and them crossing our state border," he added, citing "strategic and tactical aircraft, cruise missiles, drones, hypersonic and other flying vehicles".

It is a response to deliberations in London and Washington over whether to allow Ukraine to strike deeper inside Russia with Western missiles, including British Storm Shadows.

The comments appeared to have been deliberately timed to coincide with Ukrainian President Volodymyr Zelenskyy's address to world leaders at the UN General Assembly, as Kyiv continues to appeal for the green light on long-range missiles.

As ever, the question is - is this more rhetoric from Russia or are these threats for real?

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It's certainly not the first time Vladimir Putin has played the nuclear card. In fact, he's brandished it so many times in recent months that it seems to be losing its impact.

Red lines have been crossed by the West without consequence - providing missiles to Ukraine, then battle tanks, and most recently F-16s.

Sabre-rattling with a blunt blade.

This feels like an attempt to sharpen the sword. To redraw the red lines. Because it's slightly different from the nuclear threats before.

They were less clearly defined. This is more nuanced and specific, since it's attached to this legal framework.

But does that mean Vladimir Putin will reach for the red button at the sight of the first missile crossing the border, after these changes come into force?

That's what Joe Biden and Keir Starmer will be trying to figure out.