The United States warned Moscow of "catastrophic consequences" if it uses nuclear weapons in Ukraine after Russia promised protection to Ukrainian regions it might annex following widely criticized referendums.
Citizens in four regions of Ukraine were voting for a fourth day on Monday in the Russian-organised referendums that Kyiv and the West have branded a sham.
They say the outcomes are predetermined and even countries with close ties to Moscow, such as Serbia and Kazakhstan, say they will not recognize the results.
But by incorporating the four regions – Luhansk and Donetsk in the east and Zaporizhzhia and Kherson in the south – Moscow could portray Ukraine's 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 the United States would respond to any Russian use of nuclear weapons against Ukraine and had spelled out to Moscow the "catastrophic consequences" it would face.
"If Russia crosses this line, there will be catastrophic consequences for Russia," Sullivan told NBC television on Sunday. "The United States will respond decisively."
Sullivan did not say how Washington would respond but said it had privately told Moscow "in greater detail exactly what that would mean".
His comments followed Wednesday's thinly veiled nuclear threat by President Vladimir Putin, who said Russia would use any weapons to defend its territory.
Asked at the weekend if Moscow would consider using nuclear weapons to defend annexed regions, Foreign Minister Sergei Lavrov said Russian territory, including that "further enshrined" in Russia's constitution in the future, was under the "full protection of the state".
Ukraine's President Volodymyr Zelenskyy said he did not believe Putin was bluffing when the Kremlin leader said Moscow would be ready to use nuclear weapons to defend Russia.
The governor of Luhansk, Serhiy Gaidai, said Russian-backed officials were carrying ballot boxes from door to door, accompanied by security officials.
Residents' names were taken down if they failed to vote correctly or refused to cast a ballot, he said.
"A woman walks down the street with what looks like a karaoke microphone telling everyone to take part in the referendum," the governor said in an interview posted online.
"Representatives of the occupation forces are going from apartment to apartment with ballot boxes. This is a secret ballot, right?"
The four regions represent about 15% of Ukraine, or roughly the size of Portugal. Russian forces do not control all the territory in those regions, where fierce fighting still rages.
They would add to Crimea, an area nearly the size of Belgium annexed by Russia in 2014 after a similar referendum there.
Voting ends on Tuesday and Russia's parliament could then move swiftly to formalise the annexations.
Ukraine, bolstered by sophisticated Western weapons, has recaptured swathes of territory over the past month, prompting Putin last week to order Russia's first military mobilisation since World War Two to enlist 300,000 additional troops.
That move has unleashed protests across Russia and sent many men of military age fleeing. Almost 17,000 Russians crossed the border into Finland over the weekend, an 80% rise from a week earlier, Finnish authorities said on Monday.
More than 2,000 people have been detained across Russia for protests at the draft, says independent monitoring group OVD-Info. With criticism of the conflict banned, the demonstrations were among the first signs of discontent since the war began.
A 25-year-old gunman opened fire at a draft office on Monday in the Irkutsk region of Siberia, the local governor said.
In Russia's Muslim-majority southern region of Dagestan, police clashed with protesters, leading to the detention of at least 100 people.
Ukraine's Zelenskiy acknowledged the Russian protests in a Sunday 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."
Separately, Zelenskiy said Ukraine had discovered two more mass burial sites containing the bodies of hundreds of people in the northeastern town of Izium, part of territory recaptured from Russian forces this month.
Heavy fighting saw more than 40 towns hit by Russian shelling, Ukraine officials said on Monday.
In the 24 hours to Monday morning, Russian forces launched five missile and 12 air strikes, as well as more than 83 attacks from multiple rocket-propelled grenades, the general staff of Ukraine’s armed forces said.
More than 40 settlements in all were affected by enemy fire, mostly in southern and southeast Ukraine.
Two drones launched by Russian forces into Ukraine's Odesa region hit military objects, causing a fire and setting off ammunition, Ukraine's southern command said on Monday.
"As a result of a large-scale fire and the detonation of ammunition, the evacuation of the civilian population was organized," it said on Telegram.
"Preliminarily, there have been no casualties."
Countering Russian attacks, Ukraine’s air force launched 33 strikes, hitting 25 "enemy" areas, the general staff added.
Reuters could not independently verify the accounts.
