Ukraine has already said that any Russian vote in the Donetsk, Luhansk, Zaporizhzhia and Kherson regions will be null and void, and that it will prosecute any observers sent to monitor voting.

Moscow's ability to hold the election in what it calls its "new territories" is politically important for the Kremlin. But it raises logistical and security challenges because Russian troops only partly control the four regions.

The areas controlled by Moscow have been placed under martial law. Russia's annexation claim last year, seven months after its invasion of Ukraine, was rejected as illegal by Ukraine and most countries at the United Nations General Assembly.

President Vladimir Putin confirmed last Friday, in comments to Russian soldiers who have fought in Ukraine, that he will run again for president in the March election, where he faces no serious competition.

(Reporting by Reuters; Editing by Gareth Jones)