Russia will officially annex four regions of Ukraine occupied by its troops at a grand ceremony in Moscow on Friday, the Kremlin has announced after proposing the use of nuclear weapons to defend the areas.

Threats from senior Russian officials have not deterred a full-scale counteroffensive from Kyiv that has pushed back Russian forces to the east and is approaching the Donetsk region town of Lyman, which Moscow’s forces have battered for weeks to fully capture.

President Vladimir Putin’s spokesman, Dmitry Peskov, said the annexation will be formalized at a ceremony in the Kremlin. The Russian leader will make a “big” speech, he added.

“Tomorrow at 15:00 (1200 GMT) a signing ceremony on the incorporation of the new territories into Russia will be held in the Saint George Hall in the Grand Kremlin Palace,” Peskov told reporters.

The Kremlin-installed leaders of the four regions gathered in the Russian capital on Thursday, a day after pro-Moscow authorities appealed directly to Putin to integrate the territories into Russia.

Their near-simultaneous applications to the Kremlin came after the four regions claimed residents unanimously backed the move in hastily organized referendums dismissed as illegal, fraudulent and void by Kyiv and the West.

Ukraine, following the so-called referendums, said the only appropriate response from the West was to hit Russia with more sanctions and provide Ukrainian forces with more weapons to continue retaking territory.

“Ukraine cannot and will not tolerate attempts by Russia to conquer any part of our country,” President Volodymyr Zelenskyy said late Wednesday.

The four territories – Kherson and Zaporizhia in the south; Donetsk and Lugansk to the east – creating a crucial land corridor between Russia and the Crimean Peninsula, annexed by Moscow in 2014.

All five together make up about 20 percent of Ukraine, whose forces have been pushed back in recent weeks.

Kyiv’s army in particular has shown progress in the eastern Kharkiv region and in retaking areas in Donetsk, and military observers say Kyiv’s forces are on the verge of capturing Lyman.

“The enemy regularly makes attempts to encircle the city,” a pro-Russian official in Donetsk region Alexei Nikonorov told state television on Thursday.

“Right now our units are managing to repel all attacks,” he claimed.

Still, Russian forces are striking all along Ukraine’s front line, and officials in Kyiv said Thursday Russian bombings killed three in the Dnepropetrovsk region, five in Donetsk region and seven in the Kharkiv region.

Along with the threat to use nuclear weapons, Putin announced the mobilization of hundreds of thousands of Russian men to reinforce the Moscow army in Ukraine, prompting demonstrations and an exodus of men abroad.

On a bright morning in Ulaanbaatar, the capital of Mongolia, a young Russian fleeing Moscow’s first military operation since World War II had a straight answer as to why he left: “I don’t want to kill people.”

“It was very difficult to leave everything behind – home, home, my relatives – but it’s better than killing people,” the man, in his 20s, told AFP on condition of anonymity.

The exodus from Russia to Mongolia follows a similar trend in ex-Soviet countries like Georgia, Kazakhstan and Armenia.

EU member Finland is also affected and announced on Thursday that it would close its border to Russians with Schengen tourist visas from the next day.

Russia’s attempt to annex Ukraine’s lands was met by the United States announcing a new $1.1 billion weapons and supplies package, including precision missile systems, ammunition, armored vehicles and radars.

And the European Commission proposed new sanctions on Russian exports worth €7 billion, an oil price cap, an expanded travel blacklist and an asset freeze.