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President Vladimir Putin on Monday ordered Russia's military to act as "peacekeepers" in two breakaway regions of Ukraine, just hours after he recognised them as independent.
Mr Putin's recognition of the separatist republics as independent effectively buries a fragile 2015 peace plan for the conflict, and opens the door for direct Russian military involvement.
In two official decrees, Mr Putin instructed the defence ministry to assume "the function of peacekeeping" in the Donetsk and Luhansk regions.
Moscow provided no details or date for any deployment, with the order only saying that it "comes into force from the day it was signed".
Russia has moved tens of thousands of soldiers to regions near Ukraine's borders, with the West saying Moscow plans to use them for an attack at any moment.
In the same document, Mr Putin also ordered his foreign ministry to "establish diplomatic relations" with the "republics".
Ukrainian President Volodymyr Zelenskyy accused Russia of wrecking peace efforts and ruled out making any territorial concessions in an address to the nation in the early hours of Tuesday.
Mr Zelenskyy spoke after chairing a security council meeting and accused Russia of violating Ukraine's sovereign territory and said it could mean Moscow pulling the plug on the Minsk peace talks aimed at ending the separatist conflict in eastern Ukraine.
"We are committed to the peaceful and diplomatic path, we will follow it and only it," Mr Zelenskyy said. "But we are on our own land, we are not afraid of anything and anybody, we owe nothing to no one, and we will give nothing to no one."
He spoke after top Russian security officials convened in Moscow to review requests by the self-proclaimed Donetsk People’s Republic and Luhansk People’s Republic to be treated as independent.
Mr Putin said that Ukraine was a product of the Soviet era and had since become a US colony of puppets. He accused the country of harbouring ambitions to create a nuclear arsenal.
Western officials have said recognition would breach international law and kill off a long-stalled peace process in Ukraine.
In Washington, President Joe Biden vowed that the US and its allies would act "swiftly and decisively" after Mr Putin authorised troops to enter Luhansk and Donetsk.
Mr Biden "strongly condemned" Mr Putin’s decision to recognise the independence of Luhansk and Donetsk, the White House said after he held a call with Mr Zelenskyy.
Mr Biden "reiterated that the United States would respond swiftly and decisively, in lock-step with its allies and partners".
British Prime Minister Boris Johnson said Mr Putin’s decision to recognise the two separatist Ukrainian republics was an “ill omen” and a “dark sign” that things were moving in the wrong direction.
The German and French leaders were warned in advance of the recognition and expressed "their disappointment with this development of the situation”.
UN chief Antonio Guterres said Russia's decision amounted to "a violation of the territorial integrity and sovereignty of Ukraine and inconsistent with the principles" of the UN Charter".
French President Emmanuel Macron, who was still pressing for a diplomatic solution earlier on Monday, called for EU sanctions against Moscow.
"He is demanding an emergency meeting of the UN Security Council as well as the adoption of targeted European sanctions," Mr Macron's office said.
EU foreign policy chief, Josep Borrell, said the decision was a blatant breach and had earlier in the day threatened possible sanctions.
"We are ready to react with a strong united front in case he [Putin] should decide to do so. We expect that he will not. But if he does, we will react with a strong and united front," he said on Monday after talks with Ukraine's foreign minister.
He said Russia's troop movements gave Britain "strong cause for concern that President Putin is still committed to an invasion".
“The ripples of anything that happens in Ukraine will be felt right across Europe, whether they are in Nato or not,” he said.
Russia is widely suspected of supporting the pro-Kremlin rebels in eastern Ukraine in a war that has killed more than 14,000 people since 2014, although it denies this.
The two rebel leaders made co-ordinated appeals to Mr Putin on Monday, with Donetsk leader Denis Pushilin suggesting that Russian backing could “prevent casualties among civilians” in Ukraine.
A peace agreement brokered by France and Germany envisages partial self-government within Ukraine for the breakaway regions, but those accords have foundered and Moscow and Kiev blame each other for this.
The US previously said that any recognition of the two separatist regions would “necessitate a swift and firm response” from western powers.
“This is plainly in breach of international law, it’s a flagrant violation of the sovereignty and integrity of Ukraine," Mr Johnson said as Mr Putin's address was continuing.
“It is a repudiation of the Minsk process and the Minsk Agreements. I think it’s a very ill omen and a very dark sign.”
While that was not defined, the US and its allies have been preparing a package of sanctions that they say would be activated if Mr Putin orders an attack on Ukraine.
Hopes of a diplomatic breakthrough had earlier been raised after France said Mr Putin and Mrr Biden had agreed in principle to hold talks.
But Kremlin spokesman Dmitry Peskov dented those hopes by saying there were no concrete plans for such a summit and that it was premature to talk about them. Early gains in European stock trading were wiped out after Mr Peskov’s comments.
Russia did say that Foreign Minister Sergey Lavrov would meet his US equivalent Antony Blinken in Geneva on Thursday.
Mr Lavrov told Mr Putin that dialogue should continue despite the fact that the West was “not ready to take in our central proposals”.
The central one of these is that Ukraine and other countries in the former Soviet sphere of influence should be barred from joining Nato. Alliance leaders say Moscow has no right to demand this.
There was further concern among diplomats about the situation in Belarus, which borders Ukraine to the south, and which announced on Sunday that allied Russian troops were staying on its territory indefinitely.
“In the shadows of the Russian-Ukrainian conflict, we have seen more or less an annexation of Belarus, at least militarily, by the Russians,” said Austrian Foreign Minister Alexander Schallenberg.
“I have my doubts whether the Russians will ever pull out of Belarus again.”
Agencies contributed to this report.