The Ukrainian president has alleged his country's intelligence service has uncovered plans for a Russia-backed coup d'etat against him set for next week.
He did not give many details to back up his allegation, but President Volodymyr Zelensky said the coup involved one of Ukraine's richest oligarchs, Rinat Akhmetov.
The president said Ukrainian intelligence had recordings of an alleged meeting between Russian and Ukrainian officials discussing a plan for a coup allegedly funded by Mr Akhmetov - who is worth an estimated £5.6 billion.
"He is being drawn into this war," Mr Zelensky said.
"I believe he has begun it and I believe that this is a big mistake."
Mr Zelenskyy refused to disclose further details about the alleged coup, saying only that he does not plan to flee the country.
However, both the oligarch and the Russian government rejected the allegations.
Watch: Ukraine: NATO 'watching closely' as Russian military builds near border.
"Russia had no plans to get involved," Kremlin spokesman Dmitry Peskov said in comments to journalists in Moscow.
"Russia never does such things at all."
Mr Akhmetov called the allegations "an absolute lie".
"I am outraged by the spread of this lie, no matter what the President's motives are," Mr Akhmetov said in a statement relayed to the Associated Press by his spokeswoman, Anna Terekhova.
Asked about the alleged coup plans, the US State Department's top official for European and Eurasian affairs, Karen Donfried, said: "We are in touch with the Ukrainian government to discuss this further, and we're working to obtain additional information."
Watch: US watching Russian presence near Ukraine 'very closely'.
In recent weeks, Ukrainian and western officials have expressed concern that a Russian military build-up near Ukraine could signal plans by Moscow to invade its ex-Soviet neighbour.
The Kremlin insists it has no such intention and has accused Ukraine and its western backers of making the claims to cover up their own allegedly aggressive designs.
NATO secretary-general Jens Stoltenberg warned Russia on Friday that any attempt to invade Ukraine will have consequences.
"If Russia uses force against Ukraine, that will have costs," Mr Stoltenburg said.
"This is the second time this year that Russia has amassed a large and unusual concentration of forces in the region.
"This military build-up is unprovoked and unexplained.
"It raises tensions and it risks miscalculations."