The threats have not deterred a sweeping Ukrainian counter-offensive, which has been pushing back Russian troops in the east and is on the doorstep of the Donetsk region town of Lyman, which Moscow's forces pummelled for weeks before capturing it this summer.

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

The Russian leader himself then 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 fall of the USSR was a tragedy, and he has recently suggested Moscow should extend again its influence over the former Soviet region.

- 'Soviet collapse' -

The Kremlin-installed leaders of the four regions that pleaded to Putin for annexation this week were gathered in the Russian capital Thursday ahead of the ceremony.

Their nearly 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.

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.

US President Joe Biden said Thursday that "the United States will never, never, never recognize Russia's claims on Ukraine sovereign territory," while UN Secretary-General Antonio Guterres condemned the annexation plans as "a dangerous escalation" that "has no place in the modern world."

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.

In the south, Ukrainian forces have been wresting back territory near Kherson, and residents of recently recaptured villages described months of terror under Russian occupation.

"They robbed and humiliated us," 72-year-old Maria Syzhuk said in the village of Vysokopillya, echoing with the dull thuds of artillery from both sides -- mostly in the distance, but sometimes a little too close.

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

- 'I don't want to kill people' -

Moscow's forces are striking back along the entire front line and officials in Kyiv said Thursday that Russian bombardment had killed three 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.

Finland's Vaalimaa crossing has been flooded with new arrivals recently and has grown more precarious after Helsinki announced it would close its border from midnight to Russians holding European tourism visas for the Schengen zone.

"I just made it through, I don't know how the others will get through. It's sad, sad," Andrei Stepanov, a 49-year-old Russian, told AFP of Finland's new restrictions.

On a bright morning in Mongolia's capital Ulaanbaatar, a young Russian fleeing Moscow's first military call-up since World War II had a stark answer for why he had left: "I don't want to kill people."

"It was very difficult to leave everything behind -- home, motherland, my relatives -- but it's better than killing people," the man in his 20s told AFP, speaking on condition of anonymity.

The United States has meanwhile pledged more money to support Ukraine, with the Senate approving $12 billion in new economic and military aid as part of a stopgap budget extension.

The European Commission has proposed fresh sanctions targeting Russian exports worth seven billion euros, an oil price cap, an expanded travel blacklist and asset freezes.

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