Russian officials have alleged that a group of Ukrainian saboteurs crossed into western Russia and fired on civilians in villages, a claim that Kyiv denied while suggesting Moscow might be seeking a "false-flag" pretext to stage new attacks on Ukraine.
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Details about the March 2 incident, near the southwestern city of Bryansk, were not entirely clear.
Russian media quoted unnamed Federal Security Service officials as saying a group, which the Kremlin later called "Ukrainian terrorists," had infiltrated the Bryansk region and attacked several villages, taking hostages in the process.
Bryansk Governor Aleksandr Bogomaz said that the group had shot and killed one person and injured a child.
Russian President Vladimir Putin issued an unusually swift response, claiming that Russia had been hit by a "terrorist attack" and vowing to crush what he said was a Ukrainian sabotage group.
"They won't achieve anything. We will crush them," Putin said in comments shown on state TV.
In Kyiv, a senior adviser to Ukrainian President Volodymyr Zelenskiy called the Russian statements 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. The partisan movement in Russia is getting stronger and more aggressive," Mykhaylo Podolyak wrote on Twitter.
A group calling itself the Russian Volunteer Corps claimed responsibility for the attack in a video that urged Russians to take up arms against the government. The group, which describes itself as "a volunteer formation in the armed forces of Ukraine," did not explain what actions it took, or why.
The group is not widely known, though it is believed to be led by Denis Kapustin, a Russian-born far-right extremist who has been based in Ukraine.
The open-source investigation organization Bellingcat, which has researched right-wing extremist groups, described the Volunteer Corps as "a unit...made up primarily of anti-Putin, anti-Kremlin, Russian far-right figures active in Ukraine."
It's unclear if the group has ties with the Ukrainian military or security agencies.
A Ukrainian military intelligence official, Andriy Yusov, appeared to endorse any sort of cross-border attack, while stopping short of claiming responsibility.
"These are people who are fighting with arms against the Putin regime and those who support him...," he said. "Perhaps Russians are beginning to wake up, realize something, and take some concrete steps."
Over the past year, Russian regions close to the border have been hit regularly by unexplained explosions, drone strikes, and other apparent sabotage targeting oil refineries and military installations near the border.
Kyiv has said it reserves the right to strike at targets inside Russia but has coyly denied any responsibility.
The reports come days after Russia's Defense Ministry said anti-aircraft units shot down two drones in the southern Krasnodar and Adygea regions. The first two drones fell near an oil reserve belonging to energy giant Rosneft, causing a fire that did not reach the reserve.
Another drone was downed in the Bryansk region and another crashed down near the city of Kolomna, near Moscow.
While none of the drones caused any casualties, Putin ordered the military to step up monitoring of the border and airspace over Russian cities and towns.