President Vladimir Putin has ordered Russian nuclear deterrent forces on alert amid tensions with the West over his invasion of Ukraine.
Speaking at a meeting with his top officials, Mr 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.
Mr 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".
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"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," the Russian president said in televised comments.
The move does not mean Moscow intends to use its nuclear weapons, but its practical meaning was not immediately clear.
Russia and the United States typically have the land-based 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.
The US ambassador to the United Nations responded to the news from Moscow while appearing on CBS' 'Face the Nation'.
"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."
Soon after, the Ukrainian president's office said Ukrainian envoys will meet with Russian diplomats as Russian troops draw closer to Kyiv.
Ukrainian President Volodymyr Zelensky's office said on the Telegram messaging app that the two sides would meet at an unspecified location on the Belarusian border and did not give a precise time for the meeting.
This announcement on Sunday 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 somewhere other than Belarus, where Russia has placed a large contingent of troops.
Street fighting broke out in Ukraine's second-largest city on Sunday, day four of the full-scale invasion, and Russian troops put increasing pressure on strategic ports in the country's south following a wave of attacks on airfields and fuel facilities elsewhere which appeared to mark a new phase of Russia's invasion.