Russia has sought to defend its seven-month old war, with Foreign Minister Sergei Lavrov telling the United Nations that regions of Ukraine where widely-criticised referendums are being held would get Russia's "full protection" if annexed by Moscow.
The votes in four eastern Ukrainian regions, aimed at annexing territory Russia has taken by force since its invasion in February, were staged for a third day on Sunday (local time). The Russian parliament could move to formalise the annexation within days.
By incorporating the four areas of Luhansk, Donetsk, Kherson and Zaporizhzhia into Russia, Moscow could portray attacks to retake them as an attack on Russia itself, a warning to Kyiv and its Western allies.
The Russian annexations raise the risk of a direct military confrontation between Russia and the NATO military alliance as Western arms are being used by Ukrainian troops.
British Prime Minister Liz Truss said Britain and its allies should not heed threats from Russian President Vladimir Putin who had made what she called a strategic mistake.
"I think he didn't anticipate the strength of reaction from the free world," she told CNN.
"We should not be listening to his sabre-rattling and his bogus threats. Instead, what we need to do is continue to put sanctions on Russia and continue to support the Ukrainians," Truss said in the interview broadcast on Sunday.
Ukraine and its allies have dismissed the referendums as a sham designed to justify an escalation of the war and a mobilisation drive by Moscow after recent battlefield losses.
Putin on Wednesday ordered Russia's first military mobilisation since World War Two. The move triggered protests across Russia and sent many men of military age fleeing.
Two of Russia's most senior lawmakers on Sunday addressed a string of complaints about the mobilisation, ordering regional officials to get a handle on the situation and swiftly solve the "excesses" that have stoked public anger.
Addressing the UN General Assembly and the world's media in New York on Saturday, Lavrov sought to justify Russia's invasion of its neighbour, repeating Moscow's false claims that the elected government in Kyiv was illegitimately installed and filled with neo-Nazis.
He cast opposition to what Russia calls a "special operation" as limited to the United States and countries under its sway. Nearly three-quarters of states in the assembly voted to reprimand Russia and demand it withdraw its troops.
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Lavrov told a news conference after his speech that the regions where votes are underway would be under Moscow's "full protection" if they are annexed by Russia.
Asked if Russia would have grounds for using nuclear weapons to defend the annexed regions, Lavrov said Russian territory, including territory "further enshrined" in Russia's constitution in the future, "is under the full protection of the state".
Ukraine's foreign minister, Dmytro Kuleba, said Russia's mentions of the possible use of nuclear weapons were "absolutely unacceptable" and Kyiv would not give in to them.
The Duma, Russia's lower house of parliament, may debate bills incorporating the Russian-occupied parts of Ukraine into Russia on Thursday, the state-run TASS news agency said on Saturday, citing an unnamed source.
The Interfax agency quoted a source saying the upper house could consider the bill the same day, and RIA Novosti, also citing an unnamed source, said Putin could be preparing to make a formal address to an extraordinary joint session of both houses on Friday.
Russia says the referendums, hastily organised after Ukraine recaptured swathes of the northeast in a counter-offensive this month, enable people in those regions to express their view.
The territory controlled by Russian or Russian-backed forces in the four regions represents about 15 percent of Ukrainian territory. Adding Crimea, which Russia annexed in 2014, Russia would have gained an area about the size of the US state of Pennsylvania.
However, Russia does not fully control any of the four regions, with only around 60 percent of Donetsk region controlled by Russian or Russian-backed forces.
Ukrainian President Volodymyr Zelenskiy said on Sunday his country would regain all the territory Russia had taken.
"We will definitely liberate our entire country - from Kherson to the Luhansk region, from Crimea to the Donetsk region," he said on the Telegram messaging app.
Ukraine and Russia traded accusations on Sunday of attacks on civilians, with Ukraine's military saying that Russian forces had launched dozens of missile attacks and air strikes on military and civilian targets in the past 24 hours.
Russia used drones to attack the centre of the southern city of Odesa, Ukraine's military said. No casualties were reported.
Russia denies deliberately attacking civilians.
Its RIA state news agency reported that Ukrainian forces bombed a hotel in the city of Kherson, killing two people. Russian forces have occupied the southern city since the early days of the invasion on Feb. 24.
There was no immediate response from Ukraine.
Russia's defence ministry also said Ukrainian forces had continued attacks around the Zaporizhzhia nuclear plant in the south of the country, including launching eight "kamikaze drones" at the facility, which it controls.
Reuters was unable to verify battlefield reports.
Putin's mobilisation drive has stirred unrest in Russia. More than 2,000 people have been detained across the country for protesting against the draft, according to independent monitoring group OVD-Info.