Kremlin spokesman Dmitry Peskov told reporters that the annexations would be formalised at the ceremony and Putin would deliver a "major" speech.

It comes after foreign leaders critical of Russia voiced their opposition to the plan, with US President Joe Biden saying the United States would "never, never, never" recognise Russian sovereignty over the territories.

The Kremlin's atomic threats have not deterred a sweeping Ukrainian counter-offensive, which has been pushing back Russian troops in the east.

Kyiv's forces are on the doorstep of the Donetsk region town of Lyman, which Moscow's forces pummelled for weeks before capturing it this summer.

Putin has blamed the war in Ukraine on the West and said simmering conflicts in the former Soviet Union were the result of its collapse.

The rhetoric built on his now famous phrase that the fall of the USSR was a tragedy, and he has recently suggested Moscow should again extend its influence over the former Soviet region.

The four regions' Kremlin-installed leaders who pleaded to Putin for annexation this week assembled in the Russian capital ahead of the ceremony.

Their almost simultaneous requests came after they claimed residents had unanimously backed the move in hastily organised referendums that were dismissed by Kyiv and the West as illegal, fraudulent and void.

- 20 percent of Ukraine -

Ukraine said the only appropriate response from the West was to hit Russia with more sanctions and to supply Ukrainian forces with more weapons to keep reclaiming territory.

UN Secretary-General Antonio Guterres condemned the annexation plans as "a dangerous escalation" that "has no place in the modern world."

The UN Security Council will vote Friday on a resolution condemning the referendums, according to France, the council's current president, but it has no chance of passing due to Moscow's veto power.

Ukraine's President Volodymyr Zelensky called an "urgent" meeting of his national security council for Friday, his spokesman said, after the Kremlin announced the timing of the annexation ceremony.

The four territories -- Kherson and Zaporizhzhia in the south and Donetsk and Lugansk in the east -- create a crucial land corridor between Russia and the Crimean peninsula, annexed by Moscow in 2014.

Together, all five make up around 20 percent of Ukraine, whose forces in recent weeks have been clawing back ground.

Ukrainian troops have been progressing in the eastern Kharkiv region and recapturing territory in Donetsk. Military observers say Kyiv's forces are close to capturing Lyman.

Moscow's forces are striking back along the entire front line and officials in Kyiv said Thursday that Russian bombardment had killed three people in the Dnipropetrovsk region, five in Donetsk and wounded seven in the Kharkiv region.

Along with threats to use nuclear weapons, Putin announced a mobilisation of hundreds of thousands of Russians to bolster Moscow's army in Ukraine, sparking demonstrations and an exodus of men abroad.

Putin on Thursday called for mistakes with the draft to be "corrected", as discontent grows over the often-chaotic conscription push.

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