The self-declared people's republics of Donetsk and Luhansk are home to Russia-backed rebels who have been fighting Ukrainian forces since 2014.
Russia's move is likely to end peace talks in the region, which has been under a tenuous ceasefire for years.
Western powers also fear it could pave the way for Russian military forces to enter Ukraine's eastern regions.
UK Prime Minister Boris Johnson said it would be "a flagrant violation of the sovereignty and integrity of Ukraine" and break international law. He said it was "a very ill omen and a very dark sign".
