Russian President Vladimir Putin on Friday signed documents to incorporate four Ukrainian territories into Russia in a televised ceremony in the Kremlin.
Russia declared the annexations of the regions - Donetsk, Luhansk, Zaporizhzhia and Kherson - after holding what it called referendums in occupied areas of Ukraine.
Western governments and Kyiv said the votes breached international law and were coercive and unrepresentative.
"People living in Luhansk, Donetsk, Kherson region and Zaporizhzhia region are becoming our compatriots forever," Putin told a crowd of officials in a ceremony in an ornate hall.
"We will defend our land with all our strength and all our means," he said, calling on "the Kyiv regime to immediately cease hostilities and return to the negotiation table".
Putin's proclamation of Russian rule over 15 per cent of Ukraine - the biggest annexation in Europe since World War Two - has been firmly rejected by Western countries and even many of Russia's close allies.
UN General Secretary Antonio Guterres called it an illegal violation of the UN charter.
It comes as Russian forces have faced setbacks on the battlefield, with one of the worst so far looming even as Putin spoke.
Pro-Russian officials acknowledged that Russian troops were on the verge of encirclement in Lyman, their main garrison in the north of Donetsk province.
Defeat there could open the way for Ukraine to recapture swathes of the territory that Putin has now declared to be part of Russia.
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