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Ukraine's President Volodymyr Zelensky says Russian "representatives" are planning to overthrow his government next week, as a Russian troop build-up sparks fears of an invasion.
Mr Zelensky said audio recordings suggested Ukraine's richest businessman was being dragged into the plan, though this has been denied.
The Kremlin spokesman, meanwhile, said Russia had "no plans to get involved".
Moscow has also dismissed concerns over the troop build-up as "alarmist".
On Friday, Nato's Secretary-General Jens Stoltenberg warned that Russian use of force against Ukraine would "come at a cost", without specifying what this would be.
Mr Zelensky denounced a "very dangerous rhetoric" coming out of Russia.
"It is a signal… that there could be escalation," he said. "There is a threat today that there will be war tomorrow. We are entirely prepared for an escalation."
A large part of the Russian force is in Crimea, the peninsula which Russia annexed from Ukraine in March 2014. Troops have recently been massing near Ukraine's eastern Donbas region, the name for Luhansk and Donetsk areas partially under the control of Russian-backed separatists.
"I have received information that a coup will happen in our country on 1 December," the Ukrainian president said in a lengthy news conference.
Mr Zelensky said business magnate Rinat Akhmetov – seen as an opponent of the president – was "being dragged into war against the Ukrainian state".
Mr Zelensky did not name the alleged plotters, but said Mr Akhmetov may or may not know about such plans that he was being drawn into by his "inner circle".
In a statement, Mr Akhmetov dismissed the president's claims, saying he was "outraged by the spread of this lie".
"My position was and will be unequivocal, an independent, democratic and integral Ukraine with Crimea and my native Donbas. My actions confirm this," he added in a statement published in Ukrainian media.
The Ukrainian leader did not directly accuse the Russian government over the alleged plan, but Kremlin spokesman Dmitry Peskov responded almost immediately by saying Russia "never does such things at all".
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