It comes after Moscow-backed authorities staged votes in the occupied regions of Ukraine's east and south that were widely denounced as a sham to justify a land grab following Russia's recent military setbacks.
Kremlin spokesman Dmitry Peskov told reporters that Russian President Vladimir Putin would attend a ceremony on the accession of the four regions — Donetsk, Kherson, Luhansk and Zaporizhzhia — at the Kremlin's St. George's Hall on Friday.
"There will be a big speech by Putin there too," he said.
Ukraine’s Foreign Ministry has decried the ballots as an illegal "propaganda show." The United States and its Western allies have also vowed not to recognize Russia’s claim over the occupied areas.
The votes saw armed troops accompany election officials going door to door asking people to cast a ballot.
The State Department says it will slap further economic sanctions on Russia in response to what spokesman Ned Price called a “land grab” Wednesday.
“Based on our information, every aspect of this referenda process was pre-staged and orchestrated by the Kremlin,” White House press secretary Karine Jean-Pierre told another briefing.
The hastily arranged votes were announced last week alongside Putin's declaration that he would be calling up military reservists to bolster his struggling campaign in Ukraine.
That triggered an exodus of thousands of Russians trying to flee the draft, with miles of lines of traffic at border crossings with Georgia and Finland — the latter announcing Thursday it would close its frontier to Russian tourists.
Many Western experts see the moves as acts of desperation by Putin, whose troops have been pushed back by a lightning Ukrainian counteroffensive in recent weeks.
There is also international concern about how Moscow might seek to defend its new claimed territory if and when Kyiv tries to take it back.
In a rare national address last week to announce the partial mobilization, Putin vowed that Russia would use all the means at its disposal to protect what it considers its territory — a thinly veiled nuclear threat — and warned: "This is not a bluff.”
The U.S. has responded that any use of nuclear weapons by Russia would be met with a "catastrophic" response, without elaborating.
This is not the first time Russia has annexed Ukrainian land. In 2014, the year Moscow began supporting pro-Kremlin troops in eastern Ukraine, Putin annexed the Crimean Peninsula after a vote that was also dismissed as a sham.
Ukrainian President Volodymyr Zelenskyy says he is undeterred by the new annexations, pledging to retake and liberate all of his country's territory currently under Russian control.
Underscoring the challenges the Kremlin faces in holding onto that land despite the formal annexation, Russian troops in one strategic city in Donetsk — Lyman — are on the verge of being encircled by Ukrainian troops, according to the latest daily briefing by the Institute for the Study of War, a U.S.-based military think tank.