Russian-installed officials in four occupied regions of Ukraine reported huge majorities of votes in favour of joining Russia as the United States planned a UN resolution condemning the referendums as shams and Moscow remained defiant.
Europe was investigating what Germany, Sweden and Denmark said on Tuesday were attacks that caused major leaks from two Russian energy pipelines. But it remained far from clear who might be behind the leaks.
Hastily arranged votes took place over five days in the eastern regions of Donetsk and Luhansk and in Zaporizhzhia and Kherson to the south that together make up about 15% of Ukrainian territory.
Vote tallies from complete results on Tuesday in the four provinces ranged from 87% to 99.2% in favour of joining Russia, according to Russia-appointed officials. The head of the upper house of the Russian parliament said the chamber might consider annexation on Oct 4.
"The results are clear. Welcome home, to Russia!," Dmitry Medvedev, a former president who serves as deputy chairman of Russia's Security Council and an ally of President Vladimir Putin, said on Telegram.
Within the occupied territories, Russian-installed officials took ballot boxes from house to house in what Ukraine and the West said was an illegitimate, coercive exercise to create a legal pretext for Russia to annex the four regions.
"This farce in the occupied territories cannot even be called an imitation of a referendum," Ukrainian President Volodymyr Zelenskiy said in his nightly video address on Tuesday.
The United States will introduce a resolution at the United Nations Security Council calling on member states not to recognise any change to Ukraine and obligating Russia to withdraw its troops, US envoy Linda Thomas-Greenfield said.
"Russia’s sham referenda, if accepted, will open a pandora’s box that we cannot close," she said at a council meeting.
Russia has the ability to veto a resolution in the Security Council, but Thomas-Greenfield said that would prompt Washington to take the issue to the UN General Assembly.
"Any referenda held under these conditions, at the barrel of a gun, can never be remotely close to free or fair," Britain's Deputy UN Ambassador James Kariuki said.
Russia's ambassador to the United Nations, Vassily Nebenzia, told the meeting that the referendums were conducted transparently and upholding electoral norms.
"This process is going to continue if Kyiv does not recognise its mistakes and its strategic errors and doesn't start to be guided by the interests of its own people and not blindly carry out the will of those people who are playing them," he said.
If Russia annexes the four Ukrainian regions, Putin could portray any Ukrainian attempt to recapture them as an attack on Russia itself.
He said last week he was willing to use nuclear weapons to defend the "territorial integrity" of Russia, and Medvedev issued a new nuclear warning on Tuesday to Ukraine and the West.
But Mykhailo Podolyak, an adviser to Zelenskiy, told Reuters that Kyiv would not be swayed by nuclear threats or by the annexation votes, and would press on with plans to retake all territory occupied by invading Russian forces.
Diplomats say the nuclear sabre-rattling is an attempt by Moscow to scare the West into reducing its support for Kyiv.
For the first time Medvedev predicted that the Nato military alliance would not directly enter the Ukraine war even if Moscow struck Ukraine with nuclear weapons.
Putin said on state TV the votes were designed to protect people from what he has called the persecution of ethnic Russians and Russian speakers by Ukraine, something Kyiv denies.
"Saving people in all the territories where this referendum is being held is ... the focus of attention of our entire society and country," Putin said.
He earlier discussed with officials the mobilisation of farmers to fight in Ukraine, the latest step in a campaign he announced last week to support what Moscow calls its "special military operation" after this month's battlefield reverses.
The mobilisation drive has sent thousands of Russians rushing to cross Russian borders into neighbouring countries.
Podolyak said Ukrainians who had helped Russia organise the annexation referendums would face treason charges and at least five years in jail. Ukrainians who were forced to vote would not be punished.
Donetsk is top priority
None of the four regions is fully under Moscow's control and there has been fighting along the entire front line, with Ukrainian forces reporting more advances since they routed Russian troops in a fifth province, Kharkiv, earlier this month.
Zelenskiy said the Donetsk region in the east remained his country's - and Russia's - top strategic priority, with "particularly severe" fighting engulfing several towns.
The general staff of Ukraine's military said late Tuesday that Russian shelling hit seven towns in Donetsk including Bakhmut, Soledar and Avdiivka.
The military said 20 towns in the Zaporizhzhia region in south-central Ukraine and 35 towns and villages in the Kherson region in the south were also hit.
Leonid Pasechnik, a separatist leader of Luhansk, said on Telegram three municipal gas workers were killed and another injured by Ukrainian shelling in Brianka.
"Their goal is to kill as many civilians as possible ... and destroy the infrastructure in towns. The missiles supplied by Nato countries are killing unarmed children, women, elderly people and municipal utility workers," he said.
Reuters could not immediately verify battlefield reports.