Speaking at a meeting with his top officials, Putin asserted that leading NATO powers had made “aggressive statements” along with the West imposing hard-hitting financial sanctions against Russia, including the president himself.
Putin ordered the Russian defence minister and the chief of the military’s General Staff to put the nuclear deterrent forces in a “special regime of combat duty.”
“Western countries aren’t only taking unfriendly actions against our country in the economic sphere, but top officials from leading NATO members made aggressive statements regarding our country," Putin said in televised comments.
His order raised the threat that the tensions with the West over the invasion in Ukraine could lead to the use of nuclear weapons.
The Russian leader this week threatened to retaliate harshly against any nations that intervened directly in the conflict in Ukraine.
Russian military vehicles pushed into Ukraine's second-largest city of Kharkiv on Sunday, on a fourth day of fighting in the biggest assault on a European state since World War Two.
Oleh Sinehubov, the head of the Kharkiv regional administration, said that Ukrainian forces were fighting Russian troops in the city and asked civilians not to leave their homes.
Russia has attacked Ukrainian oil and gas facilities, sparking huge explosions. President Volodymyr Zelenskiy said Ukrainian forces were holding off Russian troops advancing on the capital Kyiv.
Zelenskiy said the night had been brutal, with shelling of civilian infrastructure and attacks on everything, including ambulances.
"This is terror. They are going to bomb our Ukrainian cities even more, they are going to kill our children even more subtly. This is the evil that has come to our land and must be destroyed," Zelenskiy said on a short video message.
The leaders of the European Commission, France, Germany, Italy, Britain, Canada and the United States wrote: "We will hold Russia to account and collectively ensure that this war is a strategic failure for Putin.”
The Kremlin sent a diplomatic delegation to Belarus, which it said was awaiting talks with Ukrainian officials. Ukraine quickly rejected the offer, saying Belarus had been complicit in the invasion.
Western allies announced sweeping sanctions against Russia, including barring some key banks from the SWIFT payments system. The move will inflict a crippling economic blow to Moscow but also hurt Western companies and banks.
European Union foreign ministers will adopt a third round of sanctions on Russia at a virtual meeting later on Sunday, chief EU diplomat Josep Borrell has said.
Turkey called Russia's invasion of Ukraine a "war" on Sunday in a rhetorical shift that could pave the way for the NATO member nation to enact an international pact limiting Russian naval passage to the Black Sea.
Zelenskiy said his nation had submitted an application against Russia to the International Court of Justice.
"Russia must be held accountable for manipulating the notion of genocide to justify aggression. We request an urgent decision ordering Russia to cease military activity now and expect trials to start next week," Zelenskiy wrote on his Twitter.
At least 198 Ukrainians, including three children, have been killed in Russia's invasion, the head of Ukraine's Health Ministry was quoted as saying. A United Nations relief agency said its estimates showed at least 64 civilians had been killed among 240 civilian casualties, but that the real numbers were likely "considerably higher".
Russian forces blew up a natural gas pipeline in Kharkiv, while Russian-backed separatists said Ukraine had blown up an oil terminal in the breakaway Luhansk province.
Kremlin-controlled energy giant Gazprom said on Sunday that Russian gas exports via Ukraine to Europe continued normally, in line with requests from customers.
Refugees continued to pour across Ukraine's western borders, with around 100,000 reaching Poland in two days. Pope Francis called for humanitarian corridors to help refugees out of Ukraine and said those who make war should not be deluded into thinking that God is on their side.
In a sea of blue and yellow flags and banners, protesters around the world expressed support for the people of Ukraine and called on governments to do more to help Kyiv, punish Russia and avoid a broader conflict.
Ireland, Italy, the Netherlands, Sweden, Finland and Denmark said they were preparing to close their airspace to Russian planes. Australia's Qantas is opting to use an alternative flight path between Darwin and London that avoids Russian airspace, while Wizz Air has temporarily suspended all flights to and from Russia.
Russian billionaire Roman Abramovich said he was giving stewardship of Chelsea to the London soccer club's foundation.
The Russian state prosecutor's office said that anyone providing financial or other assistance to a foreign state or international organisation aimed against Russia's security could be convicted of treason, facing a maximum sentence of 20 years.
"Each fact of providing financial or other assistance...will be legally evaluated," it said in a statement on the fourth day of what Russia describes as a military operation in Ukraine.
Russia unleashed a wave of attacks on Ukraine targeting airfields and fuel facilities in what appeared to be the next phase of an invasion that has been slowed by fierce resistance. The U.S. and EU responded with weapons and ammunition for the outnumbered Ukrainians and powerful sanctions intended to further isolate Moscow.
Huge explosions lit up the sky early Sunday south of the capital, Kyiv, where people hunkered down in homes, underground garages and subway stations in anticipation of a full-scale assault by Russian forces.
“We will fight for as long as needed to liberate our country,” Zelenskiy vowed.
Terrified men, women and children sought safety inside and underground, and the government maintained a 39-hour curfew to keep people off the streets. More than 150,000 Ukrainians fled for Poland, Moldova and other neighboring countries, and the United Nations warned the number could grow to 4 million if fighting escalates.
President Vladimir Putin hasn’t disclosed his ultimate plans, but Western officials believe he is determined to overthrow Ukraine’s government and replace it with a regime of his own, redrawing the map of Europe and reviving Moscow’s Cold War-era influence.