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STORY: Russian President Vladimir Putin on Friday will begin formally annexing parts of Ukraine.

Moscow's Red Square was closed to the public ahead of the ceremony, where banners read Donetsk, Luhansk, Zaporizhzhia, Kherson - Russia!

Moscow plans to annex those four Ukrainian regions, 15% of the country's total area, or the size of Hungary or Portugal.

But just how Russia would mark the annexation was unclear.

The Kremlin did not say whether Putin would attend the Red Square concert, as he did a similar event in 2014 after Russia proclaimed annexed Ukraine's Crimea region.

Spokesman Dmitri Peskov said Putin would however make a speech inside the Kremlin.

"There will be a major speech by President Putin at the ceremony. It's important to note: this is not a presidential address to the Federal Assembly. Let there be no confusion. That is a different format and one that will take place later."

The annexation was ordered after so-called referendums took place in the Russia-controlled regions, which Kyiv and the West have decried as illegitimate shams.

That comes in the midst of a war that began in February when Russia invaded Ukraine.

Russia calls the war there a "special military operation."

Ukrainian President Volodymyr Zelenskiy warned Russians that the worst of the damage was yet to come.

"The price that one person in Russia wants to continue this war will be that the entire Russian society will be left without a normal economy, without a decent life and without respect for any human values. It can still be stopped. But to stop this, we will have to stop the one in Russia who wants war more than life. Your life, citizens of Russia."

United Nations Chief, Secretary-General Antonio Gutteres, delivered his own rebuke on Thursday.

"Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions of Ukraine would have no legal value and deserves to be condemned."

"It is a dangerous escalation. It has no place in the modern world. It must not be accepted.”

Zelenskiy will meet with his defense and security chiefs for an emergency meeting on Friday, promising a strong response to the annexations.