President Joe Biden has warned Russia that the United States will not be intimidated by reckless threats after Vladimir Putin declared the annexation of four occupied regions of Ukraine.
On Friday, President Putin appeared to make a veiled threat to use nuclear weapons to defend those regions.
He said they would "forever" be Russian – but Ukraine vowed to liberate them.
Nato chief Jens Stoltenberg said the Russian move was "the most serious escalation since the start of the war".
In a speech in Moscow, the Russian leader claimed citizens in Ukraine's eastern Donetsk and Luhansk regions, as well as in Kherson and Zaporizhzhia in the south had voted to be "with their people, their motherland".
He was referring to so-called referendums held in the regions in recent days, but Ukraine and Western governments have condemned the votes as a sham.
Much of Mr Putin's speech was used to rail at the West.
He said the US had created a "precedent" by using nuclear weapons against Japan at the end of World War Two, in an apparent threat.
Mr Putin last week said his country had "various weapons of destruction" and would "use all the means available to us", adding: "I'm not bluffing."
The Kremlin has made clear that any attack on the regions claimed by Russia would be seen as an attack on Russian soil, signalling an escalation in the war.
Russia does not fully control any of the four regions, and in his speech Mr Putin did not clear define the borders.
President Biden called out his Russian counterpart's "reckless words and threats", but added that Mr Putin was "not going to scare us".
"America and its allies are not going to be intimidated," President Biden said at the White House.
He then addressed the Russian president directly, pointing his finger into the camera.
"America's fully prepared, with our Nato allies, to defend every single inch of Nato territory," he said, in reference to the Western security bloc.
"Mr Putin, don't misunderstand what I'm saying: every inch."
Shortly after, Mr Biden's top national security official said while there was a chance of Moscow resorting to nuclear weapons, there did not appear to be an imminent threat.
Ukraine launched a new, fast-track bid to join Nato soon after Mr Putin's speech.
After a crisis meeting of Ukraine's National Security and Defence Council, President Volodymyr Zelensky said Kyiv had long been a "de facto" member of the security bloc and accused Moscow of redrawing borders "using murder, blackmail, mistreatment and lies".
Mr Zelensky vowed to liberate all Ukrainian territories, including Crimea – Ukraine's southern peninsula annexed by Russia in 2014. And he also ruled out any further negotiations with Mr Putin.
Meanwhile, Mr Stoltenberg of Nato was reluctant to be drawn on the bid, saying the decision rested with the bloc's 30 members.
The alliance's members "do not and will not" recognise any of the annexed territory as part of Russia, Mr Stoltenberg told reporters, accusing Mr Putin of "irresponsible nuclear sabre-rattling".
He called the annexation a "pivotal moment" in the war.
EU Commission President Ursula von der Leyen said "the illegal annexation proclaimed by Putin won't change anything".
"All territories illegally occupied by Russian invaders are Ukrainian land and will always be part of this sovereign nation."
Turkey described the Russian move as a "grave violation" of international law.
South Korea said it did not recognise the attempted annexations, adding that Ukraine's sovereignty, territorial security and independence must be protected.
As Mr Putin spoke in Moscow, 750km (466 miles) to the south, his forces were being surrounded by Ukrainian troops in Lyman, a strategically important town in the eastern region of Donetsk.
Ukraine's military has been keen to conceal the pace of its troops' advance in the area, but one video on social media appeared to show Kyiv's forces in the centre of Yampil, just 16km (9 miles) south-east of Lyman.
And late on Friday night, Kyiv's defence ministry said it had taken the village of Drobysheve, 8km (4 miles) north-west of Lyman.
Elsewhere, Ukraine reported on Saturday morning that the director of the Zaporizhzhia's nuclear power plant – Europe's biggest – has been detained by Russians and taken "in an unknown direction". Russia occupied the plant shortly after launching its invasion on 24 February.
This comes just hours after Ukraine accused Russia of killing 30 people in a rocket strike on a civilian convoy in the city of Zaporizhzhia.
Russia blamed Ukraine for that attack – one of deadliest in recent weeks.
In another development on Friday, Russia vetoed a UN Security Council resolution which would have condemned its annexation of the four occupied regions. Moscow's ambassador, Vasily Nebenzia, complained that it was unprecedented to seek the condemnation of a permanent member of the body.
While the Kremlin's blocking of the motion was anticipated, both India and China abstained.
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