The United States warned on Sunday of "catastrophic consequences" if Moscow were to use nuclear weapons in Ukraine, after Russia's foreign minister said regions holding widely-criticized referendums would get full protection if annexed by Moscow.
Votes in four eastern Ukrainian regions, aimed at annexing territory Russia has taken by force, were staged for a third day on Sunday. The Russian parliament could move to formalize the annexation within days.
By incorporating the areas of Luhansk, Donetsk, Kherson and Zaporizhzhia into Russia, Moscow could portray efforts to retake them as attacks on Russia itself, a warning to Kyiv and its Western allies.
U.S. National Security Adviser Jake Sullivan said on Sunday the United States would respond to any Russian use of nuclear weapons against Ukraine and that it had spelled out to Moscow the "catastrophic consequences" it would face.
"If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively," Sullivan told NBC's "Meet the Press" television program.
The latest U.S. warning followed a thinly veiled nuclear threat made on Wednesday by President Vladimir Putin, who said Russia would use any weapons to defend its territory.
Foreign Minister Sergei Lavrov made the point more directly at a news conference on Saturday after a speech to the U.N. General Assembly in New York in which he repeated Moscow's false claims to justify the invasion that the elected government in Kyiv was illegitimately installed and filled with neo-Nazis.
Asked if Russia would have grounds for using nuclear weapons to defend 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 and its allies have dismissed the referendums as a sham designed to justify an escalation of the war and a mobilization drive by Moscow after recent battlefield losses.
British Prime Minister Liz Truss said Britain and its allies should not heed threats from Putin, who had made what she called a strategic mistake as he had not anticipated the strength of reaction from the West.
"We should not be listening to his saber-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 told CNN in an interview broadcast on Sunday.
Russian news agencies quoted unidentified sources as saying the Russian parliament could debate bills to incorporate the new territories as soon as Thursday. State-run RIA Novosti said Putin could address parliament on Friday.
Russia says the referendums, hastily organized after Ukraine recaptured territory in a counter-offensive this month, enable people in those regions to express their view.
The territory controlled by Russian forces in the four regions represents about 15% of Ukraine, an area roughly the size of Portugal. It would add to Crimea, an area nearly the size of Belgium that Russia claims to have annexed in 2014.
Ukrainian forces still control some territory in each of the regions, including around 40% of Donetsk and Zaporizhzhia's provincial capital. Heavy fighting continued along the entire front, especially in northern Donetsk and in Kherson.
Ukrainian President Volodymyr Zelenskyy, who insists that Ukraine will regain all its territory, said on Sunday there had been "positive results" for Kyiv in some of the clashes. "This is the Donetsk region, this is our Kharkiv region. This is the Kherson region, and also the Mykolaiv and Zaporizhzhia regions," Zelenskyy said in his nightly video address.
The general staff of the Ukrainian armed forces said in a statement on Facebook that Russia had launched four missile and seven air strikes and 24 instances of shelling on targets in Ukraine in the past 24 hours, hitting dozens of towns, including in and around the Donetsk and Kherson regions.
Reuters could not independently verify the accounts.
PROTESTS IN RUSSIA OVER DRAFT
Putin on Wednesday ordered Russia's first military mobilization 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 mobilization, ordering regional officials to swiftly solve "excesses" stoking public anger.
More than 2,000 people have been detained across Russia for protesting against the draft, according to independent monitoring group OVD-Info. In Russia, where criticism of the conflict is banned, the demonstrations are among the first signs of discontent since the war began.
In the Muslim-majority southern Russian region of Dagestan, police clashed with protesters, with at least 100 people detained.
Zelenskyy acknowledged the protests in his video address.
"Keep on fighting so that your children will not be sent to their deaths - all those that can be drafted by this criminal Russian mobilization," he said. "Because if you come to take away the lives of our children - and I am saying this as a father - we will not let you get away alive."
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