Russian President Vladimir Putin has issued a warning to the United States, signing a revised doctrine to lower the threshold for a nuclear strike just days after the Biden administration reportedly allowed Ukraine to fire American-made missiles deep into Russia.

The updated doctrine, formally known as the "basics of state policy in the field of nuclear deterrence", outlines the threats that would make Russia — the world's biggest nuclear power — contemplate using such weapons.

Russia would consider a nuclear strike if it or its ally Belarus faced aggression "with the use of conventional weapons that created a critical threat to their sovereignty and (or) their territorial integrity", according to the revised doctrine.

The doctrine also says that an attack against Russia by a non-nuclear power with the "participation or support of a nuclear power", including the use of cruise missiles and unmanned aircraft, will be seen as a "joint attack on the Russian Federation" and could trigger a nuclear response.

It also notes in a clear reference to NATO that an aggression against Russia by a member of a military bloc or coalition is viewed as "an aggression by the entire bloc".

The previous doctrine set out in 2020 said Russia might use nuclear weapons in case of a nuclear attack by an enemy or a conventional attack that threatened the existence of the state.

The Kremlin said Russia considered nuclear weapons a means of deterrence and that the aim of the updated text was to make absolutely clear to potential enemies the inevitability of retaliation should they attack Russia.

Together, Russia and the US control 88 per cent of the world's nuclear warheads.

Mr Putin's endorsement of the new nuclear deterrent policy comes on the eve of the 1,000th day since he sent troops into Ukraine, and follows outgoing US President Joe Biden's decision to let Ukraine strike targets inside Russia with US-supplied longer-range missiles.

Asked whether the updated doctrine was deliberately issued on the heels of Mr Biden's decision, Kremlin spokesperson Dmitry Peskov said the document was published "in a timely manner" and that Mr Putin instructed the government to update it earlier this year so that it was "in line with the current situation".

The move also follows Mr Putin previously warning the US and other NATO allies that allowing Ukraine to use Western-supplied long-range weapons to strike Russian territory would mean Russia and NATO were at war.

Russia has since started mass production of mobile bomb shelters that can protect against a variety of man-made threats and natural disasters including radiation and shock waves.

Ukrainian President Volodymyr Zelenskyy urged Europe to push Moscow "harder" towards what he described as a "just peace", in a speech to the European Parliament on Tuesday marking 1,000 days since the start of Russia's invasion.

Addressing EU politicians gathered in Brussels for a plenary session via video link, Mr Zelenskyy thanked the 27-nation bloc for its support during the conflict, while also spurring it to do more.

"Putin is focused on winning this war. He will not stop on his own," Mr Zelenskyy said.

The 46-year-old leader's address came as EU defence ministers meeting nearby were under pressure to align with the United States in allowing Kyiv to strike inside Russia using donated long-range missiles.

"Putin remains smaller than the united states of Europe. I urge you not to forget this, and do not forget how much Europe is capable of achieving," Mr Zelenskyy said.

The EU opened accession negotiations with Kyiv in June, setting the war-torn country on a long path towards membership.

The bloc says it has provided Ukraine with more than 120 billion euros ($195 billion) in military, humanitarian and financial aid since Russia's 2022 invasion.

"We have accomplished much, but we must not fear doing even more now," Mr Zelenskyy said.

"We must push Russia toward [a] just peace".

EU officials have been at pains to stress that Brussels will continue to back Ukraine regardless of any potential changes to Washington's foreign policy following Donald Trump's re-election as US president.

"We will continue to stand with Ukraine today, tomorrow and every day for as long as it takes," EU parliament chief Roberta Metsola said while introducing Mr Zelenskyy.

The return of Trump, who will take office in January, has fuelled fears of a reduced US commitment to European security and a curtailing of military support for Kyiv.

Wires