Russian President Vladimir Putin has recognized breakaway rebel regions in Ukraine's east as independent states, effectively ending peace talks there.
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.
Western powers fear it paves the way for Russia to enter Ukraine's east.
The extent of the mission remains unclear, but if troops cross the border it would be the first time Russian soldiers have officially entered rebel-held territory.
Russia's parliament is expected to back treaties with the separatist regions which would give Moscow the right to build military bases in Donetsk and Luhansk.
In recent years, Russian passports have been given out to large numbers of people in both territories, and Western allies fear Russia could now move military units in under the guise of protecting its citizens.
Speaking in an hour-long televised national address immediately after Monday's announcement, Putin said modern Ukraine had been "created" by Soviet Russia, referring to the country as "ancient Russian lands".
In his hour-long address on Monday, Russia's leader 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.