Vladimir Putin has approved changes to Russia’s nuclear doctrine, setting out new conditions under which the country would consider using its arsenal.

The doctrine now says an attack from a non-nuclear state, if backed by a nuclear power, will be treated as a joint assault on Russia.

The update was proposed in September and rubber stamped on Tuesday, the 1,000th day of the war with Ukraine.

It also follows Washington’s decision on Monday to allow Ukraine to fire long-range US missiles into Russia.

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Under the changes, a large attack on Russia with conventional missiles, drones or aircraft could meet the criteria for a nuclear response, as could an attack on Belarus or any critical threat to Russia's sovereignty.

Any aggression against Russia by a state which is a member of a coalition would be seen by Moscow as aggression from the whole group.

The updates expand the number of countries and coalitions, and the kinds of military threats, subject to a possible nuclear response, according to state-run news agency Tass.

Putin has threatened the use of nuclear weapons before, and Ukraine has criticised it as "nuclear sabre-rattling" to deter its allies from providing further support.

But Russian Foreign Minister Sergey Lavrov said "we strongly are in favour of doing everything to not allow nuclear war to happen".

Speaking at a press conference at the G20 summit in Rio de Janeiro, Mr Lavrov said a declaration signed by the group, which includes Russia "clearly said we want to move towards a world free of nuclear weapons".

Announcing the change, the Kremlin urged other countries to study the changes.

"This is a very important text," Kremlin spokesperson Dmitry Peskov said, according to Tass, adding "it should become a subject to a very deep analysis".

On Monday, Russia warned of "an appropriate and tangible" response to US President Joe Biden's move to let Ukraine use ATACMS missiles to strike the country.

Such an attack inside Russian territory "would represent the direct involvement of the United States and its satellites in hostilities against Russia", a foreign ministry statement said.

Mr Peskov said on Tuesday that the new doctrine was published "in a timely manner" and that Putin had requested it be updated earlier this year so that it was "in line with the current situation", AP reported.