The Kremlin says Russian President Vladimir Putin is ordering forces to maintain peace in eastern Ukraine. But it is not immediately clear whether or when troops would enter the country.
The announcement follows a day of fast-moving developments in which Putin announced the recognition of separatist regions in Ukraine. The United States and the European Union responded by saying they would impose sanctions.
Earlier, Putin recognised the independence of separatist regions in eastern Ukraine and paved the way to provide them military support — triggering threatened new sanctions from the US and Europe and further fueling fears that Russia could imminently invade Ukraine.
The carefully scripted Kremlin announcement flew in the face of European efforts for a diplomatic solution to the escalating crisis, which has brought East-West relations to a new low and jeopardised trade. US, British and EU officials called it a breach of international law.
It also came amid a spike in skirmishes in the eastern regions that Western powers believe Russia could use as a pretext for an attack on the western-looking democracy that has defied Moscow's attempts to pull it back into its orbit.
"I consider it necessary to take a long-overdue decision: To immediately recognise the independence and sovereignty of Donetsk People's Republic and Luhansk People's Republic," Putin said.
Putin justified his decision in a far-reaching, pre-recorded speech blaming Nato for the current crisis and calling the US-led alliance an existential threat to Russia. Sweeping through more than a century of history, he painted today's Ukraine as a modern construct that is inextricably linked to Russia. He charged that Ukraine had inherited Russia's historic lands and after the Soviet collapse was used by the West to contain Russia.
White House spokesperson Jen Psaki said President Joe Biden "will soon issue an Executive Order that will prohibit new investment, trade and financing" in the regions, or on anyone "determined to operate in those areas of Ukraine". She said those measures would be separate from tougher sanctions the US is preparing in case of a Russian invasion.
European leaders had urged Putin not to recognise the regions' independence. In a joint statement, EU Commission President Ursula von der Leyen and Council President Charles Michel called it "a blatant violation of international law" and said, without elaborating, that the bloc "will react with sanctions".