Russian President Vladimir Putin on Monday night ordered Russian military to "maintain peace" in Ukraine rebel regions, shortly after he recognised the independence of two separatist regions.
It happened in an emotional address on state-run television, despite warnings from the West this would trigger sweeping sanctions.
"I believe it is necessary to take a long-overdue decision, to immediately recognise the independence and sovereignty of the Donetsk People's Republic and the Lugansk People's Republic," he said.
State television then showed Putin signing mutual aid agreements with rebel leaders in the Kremlin.
At the end of an hour-long speech, Putin asked Russia's upper house of parliament, the Federation Council to "support this decision".
The Russian leader also demanded that Ukraine end military operations against pro-Moscow rebels in the eastern part of the country, or face more possible bloodshed.
"We demand an immediate end to military operations," Putin said.
"Otherwise, all responsibility for the possible continuation of bloodshed will be fully on the conscience of the regime in power in Ukraine," he added.
The West had repeatedly warned Putin not to recognise Ukraine's rebels, a move that effectively buries a fragile peace agreement regulating the conflict.
But the Russian leader ignored these pleas. He told French President Emmanuel Macron and German Chancellor Olaf Scholz shortly before his address was aired that he would recognise them.
Putin spoke for over an hour in a speech heavy with historical references questioning Ukraine's right to sovereignty and alleging the West had cheated Moscow.
"Modern Ukraine was entirely and completely created by Russia," Putin said.
EU to react with sanctions
European Union leaders denounced Russian President Vladimir Putin's decision.
"The recognition of the two separatist territories in Ukraine is a blatant violation of international law, the territorial integrity of Ukraine and the Minsk agreements," EU chiefs Ursula von der Leyen, Charles Michel and Roberta Metsola each tweeted.
"The EU and its partners will react with unity, firmness and with determination in solidarity with Ukraine," they said.
In a joint statement, von der Leyen and Michel said: "The Union will react with sanctions against those involved in this illegal act."
Just before Putin signed the decrees recognising the two pro-Russian rebel-held parts of Ukraine and signing them on to a "mutual assistance" pact with Russia, EU foreign policy chief Josep Borrell warned that sanctions would ensue.
Borrell said he would "put the sanctions on the table" for EU foreign ministers to decide whether to adopt them if Putin went ahead.
The EU has also warned it will impose unprecedented sanctions on Moscow if it invades Ukraine after it deployed an estimated 150,000 soldiers to the border.
Von der Leyen early Monday told Germany's ARD broadcaster that "if Vladimir Putin starts a war, we will respond with the most powerful lever we have: Economic and financial sanctions, because the economy is Russia's weak point".
She said: "Financial sanctions would mean that Russia would be virtually cut off from international financial markets."
Ukraine leader discusses Putin's steps with Biden
Ukrainian leader Volodymyr Zelensky spoke to US President Joe Biden on Monday before convening an urgent security council meeting to forge a response to Russia's recognition of his country's two eastern separatist regions.
The White House said Biden and Zelensky held a 35 minute call that began and ended while Russian President Vladimir Putin was delivering a marathon national address in which he justified his decision, which is likely to lead to Western sanctions.
Neither side gave details of the call, which the White House said was held on a secure line.
Zelensky tweeted that he "discussed the events of the last hours" with Biden before convening a meeting with his top defence and security ministers.
The Ukrainian leader had earlier tweeted that he had also spoken to President Emmanuel Macron of France and Germany's Chancellor Olaf Scholz "and convened the National Security and Defence Council".
Zelensky was expected to deliver his own address to the nation later Monday.
Western leaders have pledged to sanction Russia for its unilateral recognition of a part of eastern Ukraine holding up to three million people, attempting to redraw the former Soviet republic's borders for a second time since its 2014 annexation of Crimea.
Russia's decision a 'violation of Ukraine integrity': UK PM
British Prime Minister Boris Johnson on Monday slammed Russia's recognition of separatist republics as "a flagrant violation of the sovereignty and integrity of the Ukraine".
Russian President Vladimir Putin's recognition of the independence of pro-Russian separatists in Ukraine is "a repudiation of the Minsk process and the Minsk agreements," Johnson said, referring to a faltering Ukraine peace deal.
British Foreign Secretary Liz Truss said Putin's move "signals an end to the Minsk process and is a violation of the United Nations Charter.
"We will not allow Russia's violation of its international commitments to go unpunished," Truss tweeted.
Speaking at Downing Street, Johnson said that "the UK will continue to do everything we can to stand by the people of Ukraine with a very robust package of sanctions."
"They will be triggered with the first toecap of a Russian incursion or Russian invasion. But plainly what has happened is extremely bad news.
"We will be urgently talking to our friends and allies around the world, all of whom are jointly signed up with us in this package of sanctions."
"It is becoming clear that we're going to need to start applying as much pressure as we possibly can because it is hard to see how this situation improves," Johnson said, adding that he would call Ukrainian President Volodymyr Zelensky.
Putin's move could ignite a potentially devastating conflict with Kyiv's Western-backed government.
The recognition will effectively end the already shaky peace plan in the separatist conflict in Ukraine's east.
It paves the way for Russia to move in troops to protect hundreds of thousands of residents in the regions who have been granted Russian passports.
It overshadowed last-ditch diplomatic attempts to ease weeks of tensions over fears Russia has been planning an all-out invasion of its pro-Western neighbour.