Russian President Vladimir Putin on Sunday 27 February, 2022. AFP
The order means Putin wants Russia's nuclear weapons prepared for increased readiness to launch and raises the threat that Moscow's invasion of Ukraine and the West's response to it could boil over into nuclear warfare.
Amid the worrying development, the office of Ukraine's president said a delegation would meet with Russian officials as Moscow's troops drew closer to Kyiv.
Putin, in giving the nuclear alert directive, cited not only the alleged statements by NATO members but the hard-hitting financial sanctions imposed by the West against Russia, including the Russian leader himself.
Speaking at a meeting with his top officials, Putin told his defense 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.
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 specter 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.
``President Putin is continuing to escalate this war in a manner that is totally unacceptable,'' Ambassador Linda Thomas-Greenfield said. ``And we have to continue to condemn his actions in the most strong, strongest possible way.''
The practical meaning of 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 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. That would mark a worrisome escalation and a potential crisis, he said.
The alarming step came as street fighting broke out in Ukraine's second-largest city 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.
Around the same time as Putin's nuclear move, 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. The message did not give a precise time for the meeting.
The announcement came hours after Russia announced that its delegation had flown to Belarus to await talks. Ukrainian officials initially rejected the move, saying any talks should take place elsewhere than Belarus, where Russia placed a large contingent of troops. Belarus was one of the places from where Russian troops entered Ukraine.
Earlier Sunday, the Ukrainian capital, Kyiv, was eerily quiet after huge explosions lit up the 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.
``The past night was tough - more shelling, more bombing of residential areas and civilian infrastructure,`` Zelenskyy said.
Until Sunday, Russia's troops had remained on the outskirts of Kharkiv, a city of 1.4 million about 20 kilometers (12.4 miles) south of the border with Russia, while other forces rolled past to press the offensive deeper into Ukraine.