Biden vows 'severe' costs if Russia annexes Ukraine regions

US President Joe Biden has warned Russia there will be "swift and severe" costs if Moscow uses "sham" referendums to annex more of Ukraine.

"Russia's referenda are a sham –– a false pretext to try to annex parts of Ukraine by force in flagrant violation of international law," Biden said in a statement on Friday.

"We will work with our allies and partners to impose additional swift and severe economic costs on Russia," he said.

"The United States will never recognise Ukrainian territory as anything other than part of Ukraine."

White House press secretary Karine Jean-Pierre told reporters that the United States is prepared to level additional economic penalties on Russia, in lockstep with allies, if Moscow attempts to annex more Ukraine territory.

READ MORE: Five-day annexation polls open in Ukraine's Russian-held territories

Kremlin-staged vote on joining Russia

A Kremlin-orchestrated referendum is under way in occupied regions of Ukraine that sought to make them part of Russia, with some officials carrying ballots to apartment blocks accompanied by gun-toting police. 

Kiev and the West condemned it as a rigged election whose result was preordained by Moscow.

The referendums in the Luhansk, Kherson and partly Russian-occupied Zaporizhzhia and Donetsk regions are widely seen as a prelude to Moscow annexing the regions. 

The voting, which was overseen by authorities installed by Russia, is scheduled to run through Tuesday and is almost certain to go the Kremlin's way.

Moscow seized Crimea from Ukraine and annexed it in 2014 via a referundum, a move that went unrecognised by the West.

READ MORE: Will Friday’s referendum complicate the Russia-Ukraine conflict?