President Volodymyr Zelenskiy says Ukraine's intelligence service has uncovered plans to stage a coup involving people from Russia that was due to occur next week.
Zelenskiy did not give full details of the plot nor did he accuse the Kremlin of direct involvement in his comments at a press conference in Kyiv on November 26.
The Kremlin swiftly denied any role in any coup plot.
At the press conference involving journalists from Ukraine and abroad, Zelenskiy also said Ukraine was ready for any escalation from Russia amid recent reports of Russian troops massing in Russia's western regions and in illegally occupied Crimea.
Media outlets including The New York Times and Bloomberg have quoted U.S. officials as warning that Russia might attack this winter, with some saying a potential invasion could be "on a scale far greater" than in 2014, when Russia seized the Black Sea peninsula of Crimea.
On the coup allegations, Zelenskiy said he had "certain audio recordings" in which plans for carrying out such a plot on December 1 or 2 were being discussed between unspecified people from Ukraine and Russia.
He said they mentioned Rinat Akhmetov, the Ukrainian billionaire, who didn't personally take part in the conversation, according to Zelenskiy.
Zelenskiy stressed he didn't believe the billionaire would get involved in the plot, as it would be a "fatal mistake" for him to take part in the "war" against the president.
"I believe this is a setup of Rinat Akhmetov," Zelenskiy said.
Akhmetov issued a statement on Telegram calling Zelenskiy's claims "an absolute lie" and saying that he was "outraged."
Kremlin spokesman Dmitry Peskov denied the allegation of Russia being involved in the planned coup attempt.
"We never do things like that," Peskov said.
Akhmetov owns Ukraine's largest private power utility, DTEK, plus multiple businesses in eastern Ukraine, including steel and iron-ore producers, a bank, insurers, and a television channel.
In his other comments on November 26, Zelenskiy said that Ukraine was in full control of its borders and was ready for any escalation with Russia.
"There is a threat today that there will be war tomorrow," Zelensky told the press conference. "We are entirely prepared for an escalation."
Zelenskiy also said his chief of staff, Andriy Yermak, would soon be contacting representatives of Russia about the standoff between the two countries. Separately, Yermak said he would be contacting Dmitry Kozak, President Vladimir Putin's deputy chief of staff.
U.S. intelligence officials and senior figures in Ukraine's military have suggested that as many as 92,000 Russian troops are massed to the north and east of Ukraine -- many in the area around Yelnya, near Russia’s border with its ally Belarus -- and in Crimea, the peninsula that lies south of mainland Ukraine.
Amid Russia’s build-up, CNN this week quoted sources in U.S. President Joe Biden's administration as saying it was considering sending military advisers and new equipment including weapons to Ukraine.
Earlier this month, U.S. Secretary of State Antony Blinken said that Washington had "real concerns" about the Russian military moves and a possible new offensive, which he warned would be a "serious mistake."
Moscow, which also massed thousands of troops along its border with Ukraine last spring, has dismissed talk of a potential Russian invasion as "groundless."
