Ukraine has uncovered an attempted coup involving Russian citizens, claims President Volodymyr Zelenskyy.
The plot was allegedly planned to take place next week, he told reporters at a press conference on Friday.
He did not accuse Russia of state involvement. Moscow has denied any role in it.
But Zelenskyy also said that his country was "fully prepared" for a possible military escalation on its borders with Russia.
"We have to rely on ourselves, on our army, it is powerful," Zelenskyy said on Friday amid increasing tensions.
"There are intimidations that a war will take place tomorrow ... we are fully prepared for any escalation," he added.
In recent weeks, the United States, NATO, and the European Union have expressed concern about Russian troop movements around Ukraine, with Moscow denying any such plans.
Zelenskyy also said that his country faced "internal challenges" and said he had received information about a coup attempt on December 1.
According to the president's office, Kyiv has even received audio recordings of plans to involve Ukrainian business representatives in the seize of power.
Since 2014, pro-Russian separatists have been in conflict with Kyiv forces after Moscow annexed the Crimean peninsula. The war has killed more than 13,000 people.
On Sunday, Ukrainian military intelligence chiefs claimed that Russia has massed nearly 92,000 troops on Ukraine's borders and was and was preparing for an attack in January or February.
Meanwhile, the Kremlin has accused Ukraine of aggravating tensions by conducting manoeuvres near Russia's frontier.