President Vladimir Putin will sign formal documents on Friday proclaiming Russia’s annexation of four Ukrainian 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 per cent 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 phoney referendums staged at gunpoint on Russian-held Ukrainian territory, has been rejected internationally 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 recognise the annexation.

The signing 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 show 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 on 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, Peskov said. A big rock concert would be held on 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.

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.

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 threatened to use nuclear weapons to defend Russian territory if necessary.

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

Nuclear umbrella

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

Ukrainian President Volodymyr Zelenskiy has sought to rally international support against annexation in a series of calls with foreign leaders, including those of Britain, Canada, Germany and Turkey.

“Thank you all for your clear and unequivocal support. Thank you all for understanding our position,” Zelenskiy said in a late-night video address on Tuesday.

The United States has unveiled a $1.1 billion weapons package for Ukraine that includes 18 High Mobility Artillery Rocket System launchers, accompanying munitions, various types of counter-drone systems and radar systems. The announcement brings the US security aid to $16.2bn.

The United States has also said it will impose new sanctions on Russia for the referendums and the EU is expected to back a new sanctions package against Russia in the coming days.