After annexation, Moscow could deploy nuclear weapons to counter Ukrainian attempts to retake the territory, implying that Russia would use all available means to defend its territory.
Former leader Dmitry Medvedev - a Putin ally who is now deputy chairman of Russia's security council - said on Tuesday that he wanted to remind the deaf who hear only themselves: Russia has the right to use nuclear weapons if necessary.
Brigadier General Patrick Ryder, Pentagon spokesman, said the United States was taking the reiterated threat seriously but had seen nothing to cause Washington to change its nuclear posture.
The NATO chief Jens Stoltenberg said Russia must know that the war cannot be won and must never be fought. The four Russian-occupied regions of Ukraine announced that they would hold the elections just days before voting began last Friday.
They form a vital land connection between Russia and the Crimean peninsula, which Moscow annexed in 2014 and is only connected to the mainland by bridge.
The US Secretary of State Antony Blinken pledged that the West would never recognise Russian annexations of the territories, threatening Moscow with additional rapid and severe costs for its diabolical scheme. Russian Foreign Minister Catherine Colonna, in Kyiv for a surprise visit to meet Zelenskyy, called the polls a masquerade that would trigger further Western sanctions.
At the Security Council, top official Rosemary DiCarlo said that the body is fully committed to Ukraine's territorial integrity within its internationally recognised borders. The US intends to submit a resolution urging UN member states not to recognise any altered status of Ukraine and requiring Russia to withdraw its troops from Ukraine.
There is no chance that the Security Council will take a united stance on the annexation move.
Russia's UN ambassador, Vassily Nebenzia, criticised the move as temper tantrums of Western delegations The referendums were conducted only transparently, with upholding of all the electoral norms, and opening of stations in Crimea for people who fled fighting after the Russian invasion in February.
With my voice, I want to make a small contribution to stopping the war, 63-year-old Galina Korsakova from Donetsk told AFP.
The referendums followed a pattern that Moscow used in Crimea after nationwide street demonstrations saw Ukraine's Kremlin-friendly president ousted.
As in Crimea, observers saw the outcome as a foregone conclusion. In many cases, election officials brought ballot boxes door-to- door, accompanied by armed Russian forces.
According to Russian state media, the next step is to get Russia's parliament, the State Duma, to approve an annexation bill that formally incorporated the four regions into Russian territory. This could happen on Wednesday and be followed by Russian upper house approval.
According to Russian news agencies, Putin is expected to declare the Ukrainian regions part of Russia on Friday.
In the east, Ukrainian forces are pursuing their counter-offensive.
The governor of the eastern Kharkiv region said on Tuesday that its forces had recaptured Kupiansk-Vuzlovyi, one of the largest logistical and railway junctions in the region. It is not participating in this week's vote.