President Vladimir Putin vowed Russia’s annexation of four occupied regions in Ukraine is irreversible as he formalized Europe’s biggest land grab since World War II, defying efforts by Kyiv, backed by the U.S. and its allies, to recapture its lost territory.

"They will become our citizens forever,” he told officials in a Kremlin ceremony Friday before he and pro-Moscow leaders signed annexation documents. He demanded Ukraine stop fighting and begin talks, but refused to negotiate about the lands he’s absorbing.

The United Nations and many countries, including Japan, have denounced the annexation as illegal. Ukrainian President Volodymyr Zelenskyy said Thursday Russia would "annex itself to the catastrophe that it has brought to the occupied territory of our country.”