Ukraine has used US ATACMS missiles to strike Russian territory, taking advantage of newly granted permission from the outgoing administration of US President Joe Biden on the war's 1,000th day.
Russia said its forces shot down five of six missiles fired at a military facility in the Bryansk region.
Debris of one missile hit the facility, starting a fire that was swiftly put out and caused no casualties or damage, it said.
Ukraine said it struck a Russian arms depot about 110km inside Russia, an attack that caused secondary explosions.
Ukraine's military did not publicly specify the weapons used, but a Ukrainian government source and a US official confirmed it had used ATACMS.
A US official said Russia intercepted two of eight missiles and that the strike was at an ammunition supply point.
President Biden gave approval this week for Ukraine to use ATACMS, the longest-range missiles the United States has supplied, for such attacks inside Russia.
Russian Foreign Minister Sergei Lavrov said the use of ATACMS was a clear signal the West wanted to escalate the conflict.
Russia has said such weapons cannot be used without direct American operational support and their use would make the US a direct combatant in the war, prompting Russian retaliation.
The attack took place as Ukraine marked 1,000 days of war, with a fifth of its territory in Russian possession and doubts about the future of Western support as President-elect Donald Trump returns to the White House.
At the United Nations in New York, Ukraine's UN ambassador Sergiy Kyslytsya read a statement from his country, 42 others and the European Union rejecting Russia's "attempted illegal annexation" of Ukrainian territory and demanding immediate withdrawal.
"One thousand days is a tragic reminder of the need to remain resolute in ensuring ... that international law prevails, not just in Ukraine, but wherever it is challenged," he said.
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Military experts say using the US missiles can help Ukraine defend a pocket of captured Russian territory in the Kursk region as a bargaining chip, but was not likely to have a decisive impact on the 33-month-old war, in part as the move comes far too late.
The missile's range of up to 300km is much shorter than that of some Russia has used to strike Ukraine, including its hypersonic Kinzhal weapon with a reported range of up to 2,000km.
Russian President Vladimir Putin signed a new nuclear doctrine that appeared intended as a warning to the United States.
It lowers the threshold under which Russia might use atomic weapons to include responding to attacks that threaten its territorial integrity.
Mr Zelensky said the step showed Mr Putin had no interest in peace.
The Pentagon said the US State Department has approved another potential $100 million (€94 million) sale of military equipment and services to Ukraine, while Denmark said it was making a new donation of about $138 million for development of Ukraine's arms industry.
Mr Trump criticised the scale of US aid to Ukraine and said he will end the war quickly, without saying how.
Both sides appear to anticipate his return in two months will be accompanied by a push for peace talks, which are not known to have taken place since the war's early months.
The warring sides have been escalating to try to secure a stronger position at any negotiations.
Mr Zelensky said his country must do everything for the war to end diplomatically next year.
"At this stage of the war, it is being decided who will prevail," he told parliament.
He said: "Whether us over the enemy, or the enemy over us Ukrainians ... and Europeans.
"And everyone in the world who wants to live freely and not be subject to a dictator."
Dozens of civilians and soldiers gathered for a moment's silence at a candle-lit commemoration in the evening in Kyiv.
"My life has been turned upside down. There is no life for me," said Yuliia, from the city of Mariupol which was captured by Russia in 2022.
"I only want victory and to return home. I am grateful to all the men defending our country," she added.
UN spokesperson Stephane Dujarric cited data from the UN Human Rights Office that more than 12,000 civilians had been killed and nearly 27,000 injured in Ukraine over the past 1,000 days, with more than 2,400 child casualties.
Over six million Ukrainians live as refugees abroad and the population has fallen by a quarter since Mr Putin ordered the invasion that began Europe's biggest conflict since World War II.
Military losses are huge, although casualty figures remain secret.
Public Western estimates based on intelligence reports say hundreds of thousands have been wounded or killed on each side.
In the first year after the invasion, Ukrainian troops pushed Russian forces back from the outskirts of Kyiv and recaptured territory.
Since then, relentless trench warfare has ground eastern Ukrainian cities to dust.
In a move described in the West as an escalation, Russia has deployed 11,000 North Korean troops, some of whom Ukraine said have clashed with Ukrainian forces in Kursk.
Mr Zelensky said North Korea could send 100,000 soldiers.
With winter setting in, Russia on Sunday renewed its aerial assault on Ukraine's power system, launching the biggest barrage since August.
There has been no public narrowing of negotiating positions.
Russia continues Donetsk advance
After months of steady advances, Russia said that it has captured a village in Ukraine's eastern Donetsk region, as its forces also continue their push to reclaim territory in Kursk.
Its defence ministry claimed that, on the 1,000th day of Moscow's war of aggression, troops had taken Novoselydivka.
The Russian army is now rapidly advancing across the Donetsk region and closing in on the strategically important industrial hub of Kurakhove, having taken control of the surrounding territory.
Further north, Russia attacked the Sumy region overnight, gutting a Soviet-era resident building and killing at least 12 people, including a child.
Emergency services said that four people were probably trapped under the rubble.
The attack hit the dormitory in the town of Glukhiv, which had a pre-war population of around 30,000 people and lies just 10km from the Kursk region in Russia, where Ukrainian troops captured swathes of territory in a major ground offensive in August.
Since the incursion, Ukrainian forces have been steadily losing ground. They warn that Russia has amassed 50,000 troops, including North Korean forces, to drive them out.
In total, Ukraine said that Russia had launched 87 drones overnight, and that it had shot down 51.
The head of NATO warned that Mr Putin must not be allowed to prevail in the war.
"Why is this so crucial that Putin will not get his way? Because you will have an emboldened Russia on our border ... and I'm absolutely convinced it will not stop there," Mark Rutte told reporters, as he joined EU ministers for defence talks in Brussels.
Ukraine's foreign ministry has urged allies to ramp up their military support to bring about a "sustainable" end to the war.
"Ukraine will never submit to the occupiers, and the Russian military will be punished for violating international law," it said in a statement.
"We need peace through strength, not appeasement," the ministry added, referring to growing calls for Ukraine to sit down at the negotiating table with Russia to end the war.
Russia also vowed to defeat Ukraine.
"The military operation against Kyiv continues," its spokesman Dmitry Peskov told reporters, before insisting that it "will be completed".
The EU's outgoing top diplomat Josep Borrell today pressed member states to align with Washington in allowing Kyiv to strike inside Russia using donated long-range missiles. That decision by the US was immediately followed by Mr Putin's revision of his nuclear doctrine.
Kyiv demands a full Russian withdrawal from all occupied territory and Western security guarantees comparable to membership in NATO's mutual defence treaty to prevent future Russian attacks.
Russia said Ukraine must drop all ambitions to join NATO and withdraw all troops from provinces Russia says it has annexed since its invasion.
