Joe Biden has vowed the US will "never, never, never" recognise Russia's attempt to annex territory in Ukraine.
The president spoke ahead of a speech on Friday in which Vladimir Putin is expected to announce that Luhansk, Donetsk, Zaporizhzhia and Kherson are joining Russia.
The Kremlin has said the four regions voted in referendums to reunite with their former Soviet motherland but Ukraine, as well as many western allies, have called these votes a sham.
Mr Biden spoke as the US announced new sanctions on Russia as a result of the annexation.
He said: "The United States, I want to be very clear about this, will never, never, never recognise Russia's claims on Ukraine sovereign territory.”
Mr Putin signed two decrees on Thursday to recognise Zaporizhzhia and Kherson as independent territories - to put the wheels in motion for their annexation.
The UN has opposed the move, which it says violates its Charter - despite the Kremlin arguing the action should be respected.
Turkey's president Recep Tayyip Erdogan, often an ally to Mr Putin, has also called on Russia to aim to reduce tensions with Ukraine.
Mr Putin’s actions are a replica of his 2014 annexation of the southern Crimea Peninsula in 2014. On Friday morning, the Red Square was decorated with propaganda celebrating the arrival of the four regions.
An advisor of Ukraine president Volodymyr Zelensky said: "Imagine... there are tanks of the occupying army and in the houses and apartments of people who have not yet left... military men with automatic weapons are putting them to their faces and saying 'vote!'”
