Four Ukrainian regions occupied by Russia announced plans to hold a vote or referendum on whether to become part of the country on Tuesday.
In this surprising move, it was indirectly seen to pave the way for the annexation of Russia.
Pro-Russian authorities in the east, Donetsk and Luhansk, and the south, Kherson and Zaporizhzhia, will hold a five-day vote starting Friday this week.
Moscow may see this formal annexation as helping to stop the loss of territory it captured after a Ukrainian counteroffensive successfully reclaimed a large swath of territory in Kharkiv.
However, Ukraine and Western countries have voiced their opposition to not recognizing the annexation, warning the move will not prevent Ukraine from regaining its sovereign land.
White House national security adviser Jake Sullivan described the referendum as an affront to the principles of sovereignty and territorial integrity that underpin the international system.
He added that the United States would never recognize Russia's claim to any part of Ukraine if this were to happen.
Meanwhile, European Union (EU) foreign policy chief Josep Borrell warned to consider additional sanctions on Russia following the referendum.
Meanwhile, Russian President Vladimir Putin, who was initially expected to address the nation on Tuesday evening, has postponed it without giving any reason.
