President Vladimir Putin has put Russia's nuclear forces on high alert in the face of a barrage of Western reprisals for his war on Ukraine, which said it had repelled Russian ground forces attacking its biggest cities.
The United States said Mr Putin was escalating the war in a "totally unacceptable" way, amid signs that the biggest assault on a European state since World War II was not producing rapid battlefield victories, but instead generating a far-reaching and concerted Western response.
The order means Mr Putin wants Russia's nuclear weapons prepared for increased readiness to launch.
"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," Mr Putin said in televised comments.
Shortly after the development, the office of Ukraine's President said a delegation would meet with Russian officials at the Belarus border.
The practical meaning of Mr Putin's order was not immediately clear.
Russia and the United States typically have the land and submarine-based segments of their strategic nuclear forces on alert and prepared for combat at all times, but nuclear-capable bombers and other aircraft are not.
If Mr Putin is arming or otherwise raising the nuclear combat readiness of his bombers, or if he is ordering more ballistic missile submarines to sea, then the United States might feel compelled to respond in kind, according to Hans Kristensen, a nuclear analyst at the Federation of American Scientists.
Threats part of Putin's pattern
However while Mr Putin's comments could be considered cause for concern, both Ukraine's Foreign Minister Dmytro Kuleba and White House press secretary Jen Psaki indicated they were simply an intimidation tactic aimed at scaring Ukraine.
"We see this announcement, this order, as an attempt to raise the stakes and to put additional pressure on the Ukrainian delegation. But we will not give in to this pressure," Mr Kuleba said.
Ms Psaki said Mr Putin was resorting to a pattern he used in the weeks before launching the invasion of Ukraine, "which is to manufacture threats that don't exist in order to justify further aggression. The global community and American people should look at it through that prism. We've seen him do this time and time again."
Ms Psaki told ABC America that Russia was not under threat from NATO or Ukraine.
Mr Putin threatened in the days before Russia's invasion to retaliate harshly against any nations that intervened directly in the conflict in Ukraine, and he specifically raised the spectre of his country's status as a nuclear power.
The US ambassador to the United Nations responded to the news from Moscow while appearing on a Sunday news program.
"And we have to continue to condemn his actions in the strongest possible way."
Ukrainian and Russian officials to meet
Around the same time as Mr Putin's nuclear gambit, Ukrainian President Volodymyr Zelenskyy's office said on the Telegram messaging app that the two sides would meet at an unspecified location on the Belarusian border near the Pripyat River.
The message did not give a precise time for the meeting.
It was not immediately clear who would represent the two sides at the talks.
Ukrainian media said Ukraine was sending Foreign Minister Mr Kuleba's deputy to the meeting
The announcement came hours after Russia announced that its delegation had flown to Belarus to await talks.
It also came after Mr Zelenskyy spoke to Belarusian President Alexander Lukashenko, in talks he described as "very substantive".
Ukrainian officials initially rejected the idea of holding talks in Belarus.
That country was one of the places from where Russian troops entered Ukraine.
Fighting continues across Ukraine
Strreet fighting broke out in Ukraine's second-largest city, Kharkiv, and Russian troops squeezed strategic ports in the country's south, advances that appeared to mark a new phase of Russia's invasion following a wave of attacks on airfields and fuel facilities elsewhere in the country.
Kharkiv's regional governor said that Ukraine was back in full control of its second-largest city after an earlier attack by Russian forces.
"Control over Kharkiv is completely ours," Oleh Sinegubov wrote in a Telegram message.
"The armed forces, the police, and the defence forces are working and the city is being completely cleansed of the enemy."
Journalists on the ground reported that without having eyes on the entire city, it appeared Ukraine had control of Kharkiv.
Earlier on Sunday, the Ukrainian capital, Kyiv was eerily quiet after huge explosions lit up the early morning sky and authorities reported blasts at one of the airports.
Only an occasional car appeared on a deserted main boulevard as a strict 39-hour curfew kept people off the streets. Terrified residents instead hunkered down in homes, underground garages and subway stations in anticipation of a full-scale Russian assault.
Ukrainians have volunteered en masse to help defend Kyiv and other cities, taking guns distributed by authorities and preparing petrol bombs to fight Russian forces.
On Sunday the city's mayor Vitaly Klitschsko said no Russian troops were in the city and saboteurs had been captured.
"There are no Russian troops in the capital, but our military [and] law enforcement ... continue to detect and neutralise saboteurs," Mr Klitschko said.
A Russian Defence Ministry spokesman, Major General Igor Konashenkov, said Russian forces had blocked the city of Kherson on the Black Sea and the port of Berdyansk on the Azov Sea.
He said the Russian forces also took control of an air base near Kherson and the Azov Sea city of Henichesk.
Ukrainian authorities also reported fighting near Odesa, Mykolaiv and other areas.
Number of casualties remains unclear
The number of casualties so far from Europe's largest land conflict since World War II remains unclear.
On Sunday the Russian military admitted for the first time that some of its troops had been killed or wounded in Ukraine.
"There are dead and wounded among our comrades," Defence Ministry spokesman Major General Igor Konashenkov said.
He did not offer any numbers, but said Russia's losses were "many times" fewer than those of Ukraine.
Ukraine has claimed that its forces have killed 4,300 Russian troops.
Neither set of claims can be independently verified.
Ukraine's health ministry said 352 civilians, including 16 children, had been killed since the beginning of the war. It said 1,684 people have been wounded, including 116 children.
The United Nations said it had confirmed at least 240 civilian casualties, including at least 64 people killed.
But it said the real figures would be "considerably higher" because many reports of casualties remained to be confirmed.
Ukraine's UN ambassador, Sergiy Kyslytsya, tweeted on Saturday that Ukraine appealed to the International Committee of the Red Cross "to facilitate repatriation of thousands of bodies of Russian soldiers".
Laetitia Courtois, the International Committee of the Red Cross's permanent observer to the UN, said the situation in Ukraine was "a limitation for our teams on the ground" and "we therefore cannot confirm numbers or other details."
West sending military assistance to Ukraine
As Russia pushed ahead with its offensive, the West was working to equip the outnumbered Ukrainian forces with weapons and ammunition while punishing Russia with far-reaching sanctions intended to further isolate Moscow.
The US pledged an additional $US483 million in military assistance to Ukraine, including anti-tank weapons, body armour and small arms.
Germany said it would send missiles and anti-tank weapons to the besieged country and would close its airspace to Russian planes.
The US, European Union and United Kingdom agreed to block "selected" Russian banks from the SWIFT global financial messaging system, which moves money around more than 11,000 banks and other financial institutions worldwide, part of a new round of sanctions aiming to impose a severe cost on Moscow for the invasion.
They also agreed to impose "restrictive measures" on Russia's central bank.
Responding to a request from Ukraine's minister of digital transformation, tech billionaire Elon Musk said on Twitter that his satellite-based internet system Starlink was now active in Ukraine and that there were "more terminals en route."
German Chancellor Olaf Scholz, meanwhile, said his country was committing 100 billion euros ($155 billion) to a special fund for its armed forces, raising its defence spending above 2 per cent of gross domestic product.
Mr Scholz told a special session of the Bundestag the investment was needed "to protect our freedom and our democracy."
ABC/wires
