Russian President Vladimir Putin on Tuesday signed a revised nuclear doctrine declaring that a conventional attack on Russia by any nation that is supported by a nuclear power will be considered a joint attack on his country.

Putin's endorsement of the new nuclear deterrent policy comes on the 1,000th day after he sent troops into Ukraine on Feb. 24, 2022. Putin first announced changes in the nuclear doctrine in September, when he chaired a meeting discussing the proposed revisions, but the signing follows an apparent decision by the U.S. to let Ukraine strike targets inside Russia with American-supplied longer-range missiles, which was first learned in published reports on the weekend.

The signing of the doctrine, which says that any massive aerial attack on Russia could trigger a nuclear response, reflects Putin's readiness to threaten use of the country's nuclear arsenal to force the West to back down as Moscow presses a slow-moving offensive in Ukraine.

Asked about whether the updated doctrine was deliberately issued on the heels of the U.S. decision to ease restrictions on Ukraine using its longer-range missiles to strike Russia, Kremlin spokesperson Dmitry Peskov said the document was published "in a timely manner" and that Putin instructed the government to update it earlier this year so that it is "in line with the current situation."

Russia's president has previously warned the U.S. and other NATO allies that allowing Ukraine to use Western-supplied longer-range weapons to hit Russian territory would mean that Russia and NATO are at war.

Ukrainian President Volodymyr Zelenskyy gave a virtual address to European lawmakers in Brussels to mark the 1,000th day, urging them to continue to aid Ukraine's defence, and warning that Russia could draw upon thousands of more North Korean soldiers.

"The more time he has, the worse conditions become," he said.

Includes countries who support attack from a nuclear power

The updated doctrine states that an attack against his country by a non-nuclear power with the "participation or support of a nuclear power" will be seen as their "joint attack on the Russian Federation."

It doesn't specify whether such an attack would necessarily trigger a nuclear response. It mentions the "uncertainty of scale, time and place of possible use of nuclear deterrent" among the key principles of the nuclear deterrence.

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Canada supports decision to allow Ukraine to fire U.S. missiles into Russia

Defence Minister Bill Blair says Canada supports the Biden administration's recent decision to allow Ukraine to fire U.S.-supplied long-range missiles into Russian territory.

At the same time, it spells out conditions for using nuclear weapons in greater detail compared to the previous version of the doctrine, noting they could be used in case of a massive air attack involving ballistic and cruise missiles, aircraft, drones and other flying vehicles.

The wide formulation appears to significantly broaden the triggers for possible nuclear weapons use compared with the previous version of the document, which stated that Russia could tap its atomic arsenal if "reliable information is received about the launch of ballistic missiles targeting the territory of Russia or its allies."

The revised doctrine envisages that Russia could use nuclear weapons in response to aggression against its ally Belarus.

Belarus's authoritarian President Alexander Lukashenko, who has ruled the country with an iron hand for more than 30 years, has relied on Russian subsidies and support. He has let Russia use his country's territory to send troops into Ukraine and allowed the Kremlin to deploy some of its tactical nuclear weapons in Belarus.

Russia says it has captured another Ukraine town

Advancing village by village, Russia since August has recorded its fastest gains in Ukraine since the first year of the war.

Russian forces claimed the capture of the Ukrainian settlement of Novoselydivka in Eastern Ukraine, Russian state news agency TASS reported on Tuesday, citing the defence ministry.

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Ukrainians welcome chance to fire deeper into Russia

After nearly three years of bombardments, many Ukrainians welcome having the ability to strike back deeper into Russian territory as the U.S. is warned not to further escalate the war.

Meanwhile, eight people, including a child, were killed in a Russian drone attack on Ukraine's northeastern region of Sumy, Ukrainian officials said on Tuesday.

The drone attack overnight on a residential building in the small town of Hlukhiv bordering Russia also wounded 12 people, including two children, Ukraine's national police force said on the Telegram messaging app.

Ukraine's air force said it had shot down 51 drones and lost track of 30 more after Russia launched 87 drones overnight. Drones that disappear from radar have often been brought down by Ukrainian electronic defences.

Russian forces have pummelled the northeastern region of Sumy in recent months, damaging its critical and civilian infrastructure.

A missile strike there on Sunday killed 11 people, injured 89 and cut power for thousands.