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 due to declare that four occupied Ukrainian regions will join Russia.
The Kremlin says Luhansk, Donetsk, Zaporizhzhia and Kherson backed annexation in five-day referendums.
The so-called votes have been widely condemned outside Russia 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.
Tears and despair as Russian annexation looms
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.
Ukrainian President Volodymyr Zelensky will decide how to respond at an emergency meeting of his national security and defence council on Friday. Council secretary Oleksiy Danilov said “important, fundamental decisions for our country” would be taken, but gave no details.
None of the four occupied regions that Russia aims to incorporate are under its full control. It can only lay claim to 60% of Donetsk while Zaporizhzhia’s regional capital remains in Ukrainian hands.
As Ukrainians rushed to get their relatives out of occupied areas of Zaporizhzhia region, missiles hit a civilian convoy of cars on the outskirts of the main city. The Ukrainian governor said 23 civilians had been killed and another 28 wounded in the Russian attack. Two lines of cars were hit and the bodies of two women could be seen on the ground.
There are active front lines in all four regions and Ukrainian forces are reported to have encircled a strategically significant town in Donetsk that they lost during the summer. If Lyman falls it would mark a dramatic setback for the Russian leader.
In a phone call with Mr Putin, Turkey’s President Recep Tayyip Erdogan expressed his opposition to the planned annexation.
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.
