Moscow has confirmed it will formally annex the four Russian-backed separatist regions of Ukrainian on Friday, following last week's self-styled referendums.

President Vladimir Putin will sign formal documents proclaiming Russia's annexation of the four regions, as Moscow rushes to lock in territorial claims that the Ukrainian army is threatening to reverse on the battlefield.

The move, one of the legal steps Russia says will lead to the formal annexation of 15% of Ukraine's territory, confirms that Putin is doubling down on his war against Ukraine despite suffering a major military reversal this month.

The annexation, after what Kyiv and Western countries say were phony referendums staged at gunpoint on Russian-held Ukrainian territory, has been rejected in the West as an illegal seizure of land captured in war.

Washington and the European Union are set to impose additional sanctions on Russia over the plan, and even some of Russia's close traditional allies, such as Serbia and Kazakhstan, say they will not recognize the annexation.

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 – Kherson, Zaporizhzhia, Donetsk and Luhansk. Russia says the referendums were genuine and showed public support for the move.

After days of speculation over exactly how Russia would mark the annexation, Putin's spokesman Dmitry Peskov confirmed some details of the ceremony Thursday.

Agreements "on the accession of new territories into the Russian Federation" will be signed "with all four territories that held referendums and made corresponding requests to the Russian side," Peskov said.

Putin would deliver a major speech on the subject, he added.

A big rock concert will be held Friday on Moscow's Red Square, where a tribune with giant video screens has already been set up, with billboards proclaiming "Donetsk, Luhansk, Zaporizhzhia, Kherson – Russia!"

Peskov did not say whether Putin would make an appearance at the concert. He did so at a similar event in 2014 after Russia proclaimed it had annexed Ukraine's Crimea region.

Putin publicly backed the annexation plans in a speech last week in which he also announced the call-up of hundreds of thousands of Russian reservists, and warned he could use nuclear weapons to defend Russian territory if necessary.

With tens of thousands of Russian men fleeing abroad to escape Putin's military call-up, Finland shut one of the few remaining routes to Europe, saying it would no longer let Russians enter by land with EU tourist visas.

The head of the upper house of the Russian parliament has said the chamber could consider the incorporation of the four regions on Oct. 4, three days before Putin's 70th birthday.

What Russia is billing as a celebration comes after Moscow has faced its worst setbacks of the war, with its forces routed in recent weeks in the northeast.

Some military experts say Kyiv is poised to deliver another major defeat, gradually encircling the town of Lyman, Russia's main remaining bastion in the northern part of Donetsk province. Its fall could open the way for Ukrainian forces to launch attacks on swathes of territory that Russia now aims to annex.

"The situation looks increasingly precarious for Russian forces in Lyman as Ukrainian forces are about to cut them off," Carl Bildt, a former Swedish prime minister, said on Twitter.

"Another painful defeat for the Russian invasion forces is looming."

Kyiv has so far held back from disclosing details of the situation in Lyman. Russia's Defence Ministry said a day earlier that a Ukrainian offensive on Lyman had failed, with 70 Ukrainian soldiers killed.

Russian government officials have said that the four regions will fall under Moscow's nuclear umbrella once they have been formally incorporated into Russia.