Russian President Vladimir Putin has recognised breakaway rebel regions in Ukraine's east as independent states, effectively ending peace talks there, BBC reports.
The self-declared people's republics of Donetsk and Luhansk are home to Russia-backed rebels who have been fighting Ukrainian forces since 2014.
Russian troops have been ordered to perform so-called "peacekeeping functions" in both regions.
Ukraine's president accused Russia of wilfully violating its sovereignty.
In a late-night televised address to the nation, President Volodymyr Zelensky said Ukraine wanted peace, but declared that "We are not afraid" and "will not give anything away to anyone". Kyiv now needs "clear and effective actions of support" from its international partners.
"It is very important to see now who our real friend and partner is, and who will continue to scare the Russian Federation with words only," he added.
Western powers fear Putin's recognition of the rebel-held areas paves the way for Russian troops to officially enter Ukraine's east.
In recent years, Russian passports have been given out to large numbers of people in Donetsk and Luhansk, and Western allies fear Russia could now move military units in under the guise of protecting its citizens.
Speaking in an hour-long address immediately after Monday's announcement, Putin said modern Ukraine had been "created" by Soviet Russia, referring to the country as "ancient Russian lands".
He referred to Russia having been "robbed" during the collapse of the Soviet Union in 1991, accused Ukraine of being a "US colony" run by a puppet government, and alleged that people were suffering under its current leadership. He painted the 2014 protests which toppled Ukraine's pro-Russia leader as a coup.
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