Blinken and Lavrov Meet at G20 Summit: Russia-Ukraine War Live Updates

It remained unclear what happened in the village, although the incursion appeared to be over by Thursday afternoon. The group that claimed responsibility, the Russian Volunteer Corps, is led by a Russian nationalist in exile who opposes Mr. Putin’s rule and has been fighting for the Ukrainian cause.

It is unclear whether the group operates with the assent of the Ukrainian government, as it claims.

The group posted a video on Thursday of two armed men outside what appeared to be a medical building in the village of Lyubichane, in the Russian region of Bryansk, near the Ukrainian border.

“Russian Volunteer Corps came to Bryansk region to show the compatriots that there is hope, that free Russian people with weapons in their hands can fight the regime,” said a caption on the video that the organization posted on the Telegram messenger app. The video’s authenticity and details of the incident could not be independently verified.

Ukrainian intelligence officials have attempted to portray the incident as evidence of Russian divisions. “This is a sign that Russia can no longer function normally and this leads to internal destruction,” Andriy Cherniak, a representative for Ukrainian military intelligence said in a phone interview.

Mr. Putin accused the saboteurs of opening fire on civilians and children, a claim that could not be independently verified.

Mr. Putin was closely monitoring developments in the Bryansk region, Kremlin spokesman, Dmitri S. Peskov, told reporters. He added that Mr. Putin was receiving regular reports from officials in Bryansk.

Oleksiy Danilov, the head of the Ukrainian National Security and Defense Council, said in a statement on Thursday that Moscow was facing “a movement of Russian anti-fascist militias.”

There is no evidence that Russian Volunteer Corps or other Russian armed groups fighting against the Kremlin have a broad support base inside Russia.

The governor of Bryansk said earlier on Thursday that a sabotage group had entered Lyubichane and opened fire on a vehicle, killing the driver and injuring a child. Russian state news agencies issued conflicting reports about the episode throughout Thursday. They initially claimed that saboteurs took up to six people hostage, but later walked that back, reporting that local officials had no information about the taking of hostages.

Russia’s Tass state news agency said the saboteurs had already left Russia by Thursday afternoon. But Mr. Peskov said that “all measures are currently being taken to liquidate those terrorists.”

Ukrainian units have carried out sabotage operations in Russia-held territory in Ukraine throughout the war. Ukraine is also believed to have struck inside Russia on several occasions, including in December, when a drone launched from Russian soil hit a military base in the city of Ryazan. Ukrainian officials are often deliberately vague about strikes inside Russia, though they occasionally nod to them with cryptic nondenials.

Some Russian supporters of the war cited the incursion to demand an escalation of the conflict in Ukraine, which started when Mr. Putin launched his full-scale invasion last year.

“The last red lines have been erased today in the Bryansk region,” Konstantin Malofeev, an ultranationalist tycoon, wrote on Telegram. “Now anyone who even whispers about peace will be an accomplice of terrorists.”

Ivan Nechepurenko , Maria Varenikova and Carlotta Gall contributed reporting. Oleg Matsnev contributed research.