Russia has declared that saboteurs from Ukraine crossed into its territory and attacked border villages, a raid that fuelled fears of an escalation in the war as it has dragged into a second year.
A day after Thursday's purported attack, details of what happened remain scarce and conflicting theories about possible perpetrators and their goals are still swirling.
Ukrainian officials have denied involvement and a presidential aide described it as a false-flag attack used by the Kremlin to justify the war in Ukraine.
An obscure group of Russian nationalists who described themselves as part of the Ukrainian military claimed responsibility for the attack, but their status and goals remain unclear.
What did Russian and Ukrainian officials say?
Russian authorities reported the attack on the villages of Lyubechane and Suchany in the Bryansk region early Thursday, saying that several dozen saboteurs infiltrated from Ukraine, killed two civilians and planted explosives.
Russian President Vladimir Putin cancelled a scheduled trip to an event in southern Russia because of what he described as a “terrorist attack” deliberately targeting civilians.
Hours later, the Russian authorities said the intruders were pushed back into Ukraine and targeted by artillery fire.
Ukrainian presidential adviser Mykhailo Podolyak described the Russian claims as “a classic deliberate provocation,” saying that Russia “wants to scare its people to justify the attack on another country and the growing poverty after the year of war”. But Podolyak also alleged that the attack could be the work of Russian guerrillas who had rebelled against the Kremlin.
“The partisan movement in the Russian Federation is getting stronger and more aggressive,” he said.
Ukraine's military intelligence representative, Andrii Cherniak, similarly denied Ukraine's involvement while also alleging that Russia is facing an uprising among its own disgruntled people.
“This was done by the Russians, Ukraine has nothing to do with it,” he told AP.
Mr. Cherniak noted that a group calling itself the Russian Volunteer Corps had claimed responsibility for the attack.
What is the Russian Volunteer Corps
The Russian Volunteer Corps released a video featuring its members standing outside a post office in one of the villages and urging the Russians to rebel against Mr. Putin.
The group has described itself as “a volunteer formation" of Ukraine's armed forces. Little is known about the group, the number of its members and its ties, if any, with the Ukrainian military.
Russian bloggers identified some of the men who appeared in the video filmed in the village of Lyubechane as former members of Russia's radical nationalist groups who had moved to Ukraine several years ago.
Ukrainian New voice-NV news portal quoted Ilya Bogdanov, who identified himself as a member of the Corps, confirming that his colleagues who crossed into the Bryansk region were serving in the Ukrainian army.
Ukrainian military analyst Oleh Zhdanov said that the Corps' claim could be a Ukrainian propaganda effort intended to embarrass Russia.
“It's quite possible if our propagandists believe it would be more efficient to cast it as a heroic feat and pretend that there is an entire corps of them,” he told the AP.
Mr. Zhdanov noted that despite its flashy name, the group could include just a handful of Russians who signed a contract to fight alongside the Ukrainian military.
What do experts say?
Security analysts say it's hard to figure out quickly who was behind the attack.
Mark Cancian, a senior adviser for the International Security Program at the Centre for Strategic and International Studies, argued that Ukraine could have launched the attack to force Russia to pull back some of its forces from the front line to tighten the border.
“If I had to bet I would say it's the real thing,” Cancian said. “I can see why Ukraine might want to do this. Most of the border is not contested at the moment, so Ukraine might want to be forcing Russia to guard more of its borders, maybe pull some forces out of the Donbass.”
Eleonora Tafuro, a Russia expert at the ISPI think tank in Milan, said it appears possible the attack was carried out by the Russian Volunteer Corps to foment a sense of insecurity among the local population.
“The area is very exposed to fighting,'' she said. “It could be a message: You are vulnerable. You are exposed.'”
Brad Bowman, senior director of the Centre on Military and Political Power at the Foundation for Defence of Democracies, warned against quickly rushing to conclusions, noting that the Kremlin could be interested in rallying the public as the war drags on.
“The Kremlin's information warfare efforts are meant to deceive Russians so they will believe that Russia is under grave threat and will be willing to fight and die in an illegal war of aggression,” he said.
And William Courtney, who served as ambassador to Kazakhstan and Georgia and is now a senior fellow at the RAND Corp., a non-profit research institute, argued that the purported attack could be a false-flag operation.
“It has an engineered quality to it that was carried out to make Ukraine look like a terrorist state,” Courtney said.
What are the consequences of the attack?
The purported attack came as an embarrassment for Mr. Putin, who had told officials to tighten protection of the long and porous border with Ukraine earlier in the week.
It has caused outrage among Russian hawks, who harshly criticised the Kremlin for failing to protect the border and mount a quick and forceful retaliation.
Yevgeny Prigozhin, the rogue millionaire who owns the Wagner Group military company, mocked the authorities for idly watching the crossing of another Russian red line. And Ramzan Kadyrov, the regional leader of Chechnya, has challenged the Kremlin to up the ante by introducing martial law.
Hawkish commentators and military bloggers have derided the Kremlin's indecision, calling for strikes on Ukraine's presidential office and the deployment of hit men to target top Ukrainian officials.
It remains unclear if Mr. Putin could use the incident to double down.
In his initial statement, the Russian leader cast the purported attack as proof that Russia did the right thing by invading Ukraine, but he didn't signal an intention to change the status of the operation or ramp up strikes.
On Friday, Mr. Putin had a video call with members of his Security Council, saying in opening remarks that it would focus on tightening protection against terrorist attacks but giving no details.