Moscow's planned annexation of 15% of Ukraine's territory, after what Kyiv and Western countries say were phoney referendums staged at gunpoint in Russian-occupied areas, shows Putin doubling down on his war despite suffering a major reversal on the battlefield this month and discontent at home over a widely criticised partial military mobilisation.
Putin said that "all mistakes" made in a call-up to reinforce Russia's military operation in Ukraine should be corrected, his first public acknowledgment that the "partial mobilisation" he announced last week had not gone smoothly.
He spoke late on Thursday, on the eve of a planned ceremony in the Kremlin and concert in Moscow's Red Square to mark the annexation, details of which sparked a fresh round of international condemnation.
"Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions of Ukraine would have no legal value and deserves to be condemned," United Nations Secretary General Antonio Guterres told reporters.
U.S. Secretary of State Antony Blinken said Russia's votes were a sham and a "futile attempt to mask attempted land grab in Ukraine." Turkey's President Tayyip Erdogan pressed Putin in a call to take steps to reduce tensions in Ukraine.
Russia says the referendums were genuine and showed public support.
Ukrainian President Volodymr Zelenskiy, who will hold an emergency meeting with security and defence chiefs on Friday, promised a harsh response to a step he says has killed off chances of reviving peace talks.
The votes "are worthless and do not change reality. The territorial integrity of Ukraine will be restored," he said.
Ukraine's top general said he and the top U.S. Army commander in Europe had discussed the war and that he had received assurances of further support from Washington.
Ceremony and concert
Putin's annexation ceremony will be held in one of the Kremlin's grandest halls with the pro-Russian figures Moscow considers to be leaders of the four Ukrainian regions.
Putin's spokesman Dmitry Peskov added Putin would deliver a major speech. He did not say whether Putin would attend the Red Square concert, as he did a similar event in 2014 after Russia proclaimed it had annexed Ukraine's Crimea region.
A tribune has already been set up on the Moscow square with giant video screens and billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!"
A political adviser to the Ukrainian president said on Thursday the ceremony "does not make legal sense" and would be a "Kremlin freak show."