Moscow claimed that a group of Ukrainian saboteurs crossed into Russia and opened fire on civilians on Thursday, an allegation denied by Kyiv as a "deliberate provocation."
While Russia claims its regions bordering Ukraine are routinely shelled by Ukrainian forces, the alleged incursion, if confirmed, would be a rare instance of fighting inside its territory.
Shortly after Russia reported the alleged incursion, President Vladimir Putin called the attackers "neo-Nazis and terrorists ... who today committed another terrorist attack, penetrated the border area and opened fire on civilians."
Bryansk region Governor Alexander Bogomaz said a sabotage and reconnaissance group from Ukraine entered the village of Lyubechanye and fired at a moving car, killing one person and injuring a 10-year-old child who was hospitalized.
"In the Bryansk region's border district of Klimovsky, the FSB of Russia and additional forces of the Russian Defense Ministry are taking measures to destroy armed Ukrainian nationalists who violated the state border," Russia's Federal Security Service (FSB) said.
Civilian casualties were reported in the clashes that ensued, the state-run TASS news agency reported, citing law enforcement agencies.
The saboteurs were believed to be holding civilians hostage, TASS reported.
However, a different account was put forward in two videos on social media. They showed four men in military uniform claiming to be from a Russian volunteer group in the Ukrainian army. Among them was Denis Nikitin, a notorious Russian neo-Nazi.
The men said they were in Russia and made statements against the Russian government, denying reports that they had taken hostages or killed anyone.
Hours later, the FSB released a new statement saying that the situation "is under the control of law enforcement agencies. An inspection of the area is being carried out, and a large number of explosive devices of various types are being demined."
The FSB the "Ukrainian nationalists" had been pushed back over the border and targeted with a "massive artillery strike."
Putin had been receiving "constant" updates on the situation, the Kremlin said.
"Our soldiers and officers... protect against neo-Nazis and terrorists... those who today committed another terrorist attack, penetrated the border area and opened fire on civilians," Putin said via video link at a televised education meeting in southern Russia.
"The story about [a] sabotage group in Russia is a classic deliberate provocation. Russia wants to scare its people to justify the attack on another country and the growing poverty after the year of war," Ukrainian presidential adviser Mykhaylo Podolyak said on Twitter.
Ukraine's command in the north on Feb. 23 had warned that Russia may be planning a false-flag operation in the area, "most likely ... to accuse the Ukrainian defenders of violating territorial integrity."
Ukraine's military intelligence meanwhile described the reports of clashes in the Bryansk region "a continuation of the transformation of Russia, its purification and liberation from Putin's dictatorship," according to the independent Hromadske news outlet.
Elsewhere in the Bryansk region, a residential building in the village of Sushany caught fire after being struck by a drone, Bogomaz said, while two houses were damaged by mortar fire in the village of Lomakovka.
The governor of another Russian border region, Kursk, Roman Starovoyt, said Ukrainian shelling on the village of Tetkino had left one dead and one injured.
He said the shelling damaged three houses and cut power in the village.
AFP contributed reporting.