Voting has opened in four occupied areas of Ukraine in referendums on joining Russia, officials backed by Moscow have said.
The polls, which have been denounced as a sham by the West, are expected to lead to the Russian annexation of Luhansk, Donetsk, Kherson and Zaporizhzhia.
“We cannot — we will not — allow President Putin to get away with it,” US Secretary of State Antony Blinken said in a UN Security Council session on Thursday.
“The very international order we've gathered here to uphold is being shredded before our eyes … [Defending Ukraine] is about protecting an international order where no nation can redraw the borders of another by force.”
The polls are reminiscent of one in 2014 that led to Ukraine's Crimean Peninsula being annexed by Russia and was denounced as fraudulent in the West.
Balloting in the four areas will continue for the next five days, through to Tuesday.
Russian-installed officials said during the first four days ballots will be brought to people’s homes. People can also vote at makeshift polling stations near residential areas, they said. On the fifth day, Tuesday, voters will be invited to regular polls.
In the eastern Donetsk and Luhansk regions, which were already recognised as independent by Mr Putin before he launched the invasion in February, residents are answering if they support their “republic's entry into Russia”, according to Russian news agency Tass.
Ballots in the Kherson and Zaporizhzhia regions have the question: “Are you in favour of secession from Ukraine, formation of an independent state by the region and its joining the Russian Federation as a subject of the Russian Federation?”
Russian news agencies reported that the voting process, which is using traditional paper ballots, began on Friday morning.
Denis Pushilin, separatist leader of Moscow-backed authorities in the Donetsk region, called the referendum on Friday “a historical milestone”.
But Ukrainian President Volodymyr Zelensky denounced the polls as a “farce”, and praised Western allies' condemnation of moves by Russia.
“I am grateful to everyone in the world who supported us, who clearly condemned another Russian lie,” he said during his daily address on Thursday.
The votes are all but certain to go Russia’s way, giving it the pretext to claim Ukrainian attempts to take back the areas are an attack on Russia itself.
They follow Mr Putin’s announcement of the partial mobilisation of 300,000 reservists to fight in Ukraine.
Western leaders unanimously denounced the referendums and call-up during meetings at the UN headquarters in New York this week.
French President Macron told the UN General Assembly that the referendums were a “travesty”.
Videos shared on social media purported to show hundreds of Russian citizens across the country responding to the military summons, while the Russian military claimed at least 10,000 people had volunteered to fight in 24 hours since the order.
But men were also leaving Russia in droves before they were made to join. Across Russia on Wednesday, more than 1,300 people were arrested during protests, a monitoring group reported.
Flights to neighbouring countries, mainly former Soviet republics that allow Russians visa-free entry, are nearly entirely booked and prices have shot up, pointing to an exodus of Russians wanting to avoid going to war.
Mr Putin has said Moscow will use “all means” to protect its territory. Former Russian leader Dmitry Medvedev said on social media that this statement meant “strategic nuclear weapons” would be included.
Mr Medvedev also predicted the voting regions “will integrate into Russia”.