The US will "never, never, never" recognise Russia's attempt to annex territory in Ukraine, President Joe Biden has said.
He was speaking ahead of a speech on Friday from Vladimir Putin, who is expected to declare that four Ukrainian regions are joining Russia.
The Kremlin says Luhansk, Donetsk, Zaporizhzhia and Kherson voted to join Russia in recent referendums.
But Ukraine and the West have dismissed the votes as a sham.
The US will impose new sanctions on Russia as a result of the annexation.
"The United States, I want to be very clear about this, will never, never, never recognise Russia's claims on Ukraine sovereign territory," Mr Biden said.
On Thursday, the Russian president signed two decrees recognising Zaporizhzhia and Kherson as independent territories - paving the way for them to be annexed.
The documents, shared on Russian state media, say the independence of the two regions is being recognised in accordance with international law and "enshrined in the Charter of the United Nations".
However, UN Secretary General António Guterres has said any annexation of a country's territory based on the use of force violates the UN Charter and international law.
It is a "dangerous escalation" that "has no place in the modern world", he said.
In a phone call with Mr Putin, Turkey's President Recep Tayyip Erdogan also expressed his opposition to the plan.
He called on the Russian leader to reduce tension and give peace negotiations with Ukraine another chance, according to a spokesperson.
Turkey, along with the UN, has mediated in negotiations between Russia and Ukraine in the past - with success in reaching a deal to resume exports of grain through the Black Sea.