Western leaders are critical of plans for voting by Ukrainian separatists to become integral parts of Russia, as Moscow loses ground in the war.
The announcements of "sham" referendums starting on Friday in the Donetsk, Luhansk, Kherson and partly Russian-controlled Zaporizhzhia regions came after a close ally of Russian President Vladimir Putin said votes were needed.
Former president Dmitry Medvedev said folding regions into Russia itself would make redrawn frontiers "irreversible" and enable Moscow to use "any means" to defend them.
Ukrainian foreign minister Dmytro Kuleba denounced the votes as a sham and tweeted that "Ukraine has every right to liberate its territories and will keep liberating them whatever Russia has to say".
US national security adviser Jake Sullivan said: "We will never recognise this territory as anything other than part of Ukraine," adding that they reflect Russia’s setbacks on the battlefield.
"These are not the actions of a confident country. These are not acts of strength," he said.
German Chancellor Olaf Scholz said: "It is very, very clear that these sham referendums cannot be accepted."
The votes, in territory Russia already controls, are expected to go Moscow's way but are unlikely to be recognised by western governments backing Ukraine with military and other support.
Pressure within Russia and from Moscow-backed leaders in Russian-controlled regions of Ukraine for votes to pave their way to becoming Russian increased after a Ukrainian counteroffensive, bolstered by western-supplied weaponry, that is recapturing large areas of Russian-occupied territory.