The governor of the Russian province of Bryansk, adjacent to the Ukrainian border, announced that two people were killed, in a shooting carried out by what he described as a Ukrainian sabotage group.
The Russian president condemned the attack, describing it as terrorist.
For his part, a spokesman for the Ukrainian military administration of the Odessa region described the accusations of infiltration of Ukrainian forces into the Bryansk region as lies.
Russian President Vladimir Putin called a special meeting of the National Security Council following reports from the intelligence service of fighting with Ukrainian units on Russian soil in Bryansk province, and his planned visit to the Caucasus on Friday was abruptly cancelled.
No decisions have been announced for the National Security Council meeting, but speculation has prevailed for some time that Russia may officially declare war on Ukraine and order an additional mobilization of hundreds of thousands of personnel for the army.
When asked by reporters if the meeting of senior defense officials would escalate what Moscow calls a "special military operation" in Ukraine, Kremlin spokesman Dmitry Peskov said: "I don't know, I can't say."
Bryansk process
And Russian media reported that a group of Ukrainian "saboteurs" infiltrated the Klimovsky district in Bryansk province, and took a number of civilians hostage in a shop in a village, before Russian security forces reimposed control and security in the area.
And the Russian State Security Service had previously announced that his country's forces had clashed with a Ukrainian "sabotage group" that had infiltrated Russian territory and entered Bryansk province, and local media reported that they numbered about 50 armed men.
Russian press reports said that the group is holding more than 100 people hostage inside homes in Bryansk, and it also talked about taking a number of hostages inside a shop in the village of Lyubechany in the province.
On the other hand, the Ukrainian side denied the infiltration of armed men into Russian territory, and accused Moscow of being behind these events.
"The story of the sabotage group in Russia is a deliberate provocation aimed at intimidating Moscow for its people," said Ukrainian presidential advisor Mykhailo Podolyak.
Medvedev warned Kiev's allies against providing it with fighters (Reuters)
Russian warning
Former Russian President Dmitry Medvedev warned the North Atlantic Treaty Organization (NATO) against supplying combat aircraft to Ukraine and conducting maintenance in Poland, saying that this is tantamount to entering a direct war against Russia.
"Anyone who decides to hand over (or repair) such equipment or means of destruction, as well as provide foreign mercenaries and military trainers, should be considered a legitimate military target," Medvedev wrote on the Telegram app on Thursday.
He explained that this may be the only thing that prevents them from being described as "people with childish behavior in the West" from supplying aircraft and long-range weapons.
Medvedev, who is currently deputy secretary of the Russian Security Council, said he did not expect this to continue for a long time "because of the great desire to destroy Russia."
Kiev had asked its allies to provide it with fighters after it received Western promises to supply it with modern tanks, especially the American "Abrams" tanks and the German "Leopard 2", in addition to long-range weapons.
French President Emmanuel Macron did not rule out supplying combat aircraft to Kiev, but Western leaders expressed reservations about the matter.
Bakhmut battles
The military command in the Ukrainian capital, Kiev, said that the defenders of the eastern city of Bakhmut managed to repel several Russian attacks on Thursday.
In its report on the situation, the Kiev General Staff reported that Russian artillery had shelled a number of smaller areas around Bakhmut.
It is noteworthy that the Russian forces are closing in on the city from three sides and have been trying to encircle the entire city of Bakhmut for weeks.
Last Monday, Ukrainian President Volodymyr Zelensky said that the situation had become more complicated for the Ukrainian defenders of the besieged city of Bakhmut.
Over the past weeks, there has been ambiguity about the situation in Bakhmut, with conflicting statements from both sides regarding the course of the battles.
Meanwhile, the Wagner militia announced that its fighters had taken control of several towns and neighborhoods in and around Bakhmut, after months of battles.