KYIV, Ukraine -- Ukrainian President Volodymyr Zelenskyy asserted Friday that a group of Russians and Ukrainians planned to attempt a coup in Ukraine next month and that the plotters tried to enlist the help of the country's richest man, Rinat Akhmetov.
Zelenskyy, speaking at a "press marathon" for local and international media, said audio recordings obtained by Ukraine's security services caught plotters discussing their plans and mentioning Akhmetov's name.
Akhmetov was not involved in the purported coup plot, however, Zelenskyy said.
The president offered no other details, leaving many questions about his motives for making the allegations public and what possible actions authorities have taken.
Akhmetov, a mining and steel tycoon, also owns media holdings, which in recent weeks have increased their criticism of Zelenskyy and his administration.
A statement attributed to Akhmetov called Zelenskyy's claims "an absolute lie." Kremlin spokesman Dmitry Peskov denied any Russian role in an alleged plot.
"Russia never engages in such things. There have never been such plans," Peskov said.
Zelenskyy's comments also come against a backdrop of rising tensions between Kyiv and Moscow.
Western and Ukrainian officials say they have observed a buildup of Russian forces on the country's border with Ukraine.
The reasons for the buildup are unclear, but U.S. and other officials say it could be preparation for an invasion or an escalation in the 7-year-old conflict in eastern Ukraine with anti-Kyiv insurgents, backed by Moscow, according to Western officials and independent researchers.
Speaking in Nantucket, Mass., President Joe Biden told reporters that he would "in all probability" have talks with Zelenskyy and Russian President Vladimir Putin over the deepening tensions.
"I believe [Akhmetov] is being dragged into the war against Ukraine," Zelenskyy said. "This will be a big mistake, because it is impossible to fight against the people, against the president elected by the people of Ukraine."
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Zelenskyy said the coup was being planned for Dec. 1 or 2. He gave no further details.
Ukrainian media in recent weeks have commented on growing tensions between Zelenskyy and Akhmetov. Zelenskyy has launched a "de-oligarchization" campaign to reduce the political influence of Ukraine's richest people, who control key sections of the economy.
"I am outraged by the spread of this lie, no matter what the president's motives are," Akhmetov was quoted as saying. "My position has been and will be explicit and definite: an independent, democratic, and united Ukraine with the Crimea and my home region, Donbas."
Donbas is part of the breakaway region in eastern Ukraine. The Crimean Peninsula was annexed by Russia from Ukraine in 2014.
The Ukrainian president said his country was prepared for any scenario in the volatile east.
"We are in full control of our borders and ready if there will be an escalation," Zelenskyy said.
He added, however, that the number of Russian forces was smaller than during a buildup in the spring this year, an episode that also caused concern about a possible Russian offensive.
Zelenskyy said Ukraine had received promises of support "in public and not in public" from its Western partners if Russia were to take military action, but he did not provide details.
He said the risk of escalation or invasion from Russia was always present and that Ukraine had observed a sharp rise in aggressive comments coming from Russia, which he said was worrying.
"When Russia says it is defending its borders, it is very dangerous," Zelenskyy said.
NATO Secretary-General Jens Stoltenberg warned Russia on Friday that any attempt to invade Ukraine would have costs, as concern mounts about a Russian military buildup near its former Soviet neighbor's borders.
Moscow denies that it's planning any invasion and refuses to provide details about troop movements on its own territory.
"If Russia uses force against Ukraine that will have costs, that would have consequences," Stoltenberg said, ahead of a meeting of the 30-nation military organization's foreign ministers in Latvia on Tuesday-Wednesday, where Russia's activities will be high on the agenda. He did not say what those costs would be.
"This is the second time this year that Russia has amassed a large and unusual concentration of forces in the region," Stoltenberg told reporters. He said it includes tanks, artillery, armored units, drones and electronic warfare systems, as well as combat-ready troops.
"This military buildup is unprovoked and unexplained. It raises tensions, and it risks miscalculations," Stoltenberg said. He conceded that "there is no certainty about the intentions of Russia" but said "this is a military buildup by a country that has invaded Ukraine before."
U.S. national security adviser Jake Sullivan on Friday spoke with the head of the presidential administration in Ukraine, Andriy Yermak. The two discussed their shared concerns about ongoing Russian military activities near Ukraine's border and agreed that all sides should pursue diplomatic efforts to de-escalate tensions, National Security Council spokesperson Emily Horne said in a statement.
"Mr. Sullivan underscored the United States' unwavering commitment to Ukraine's sovereignty and territorial integrity," the statement read.
Information for this article was contributed by David L. Stern and Robyn Dixon of The Washington Post; and by Lorne Cook and Samuel Petrequin of The Associated Press.