The Belgorod border raid has thrust two groups of largely unknown fighters with neo-Nazi ties into the heart of the conflict.
Russia has blamed the attack on Kyiv, but Ukraine has denied any involvement.
As the chaos and confusion subsides, the two militant groups have emerged from relative obscurity to claim responsibility, taunt the Kremlin and promise more to come.
NBC News looks at who and what appears to be behind the raid.
What do we know about the groups?
Two groups claiming to be Russians fighting on behalf of Ukraine say they carried out the attack. Both describe themselves as Russian dissidents and have members who espoused white nationalist views.
The Russian Volunteer Corps, or RVC, says it’s made up of Russians fighting on Ukraine’s side and against Putin's government. The group made headlines in March when it claimed to be behind a smaller raid in the Bryansk region.
Its commander, Denis Kapustin, who also goes by the last name of Nikitin, is a white nationalist and ex-soccer hooligan who shares neo-Nazi views.
Kapustin is listed on Russia’s federal wanted list and its register of extremists and terrorists.
The U.S.-based Anti-Defamation League has described Kapustin as “a Russian neo-Nazi who lived in Germany for many years.” He goes by the call sign “White Rex” and reportedly founded a clothing brand of the same name that is popular among Russian neo-Nazis.
The RVC’s channel on the Telegram messaging app has more than 110,000 followers and has shared photos of what it says are the group’s fighters on the war's front lines.
Some of their posts have anti-immigration and pro-white European rhetoric. In a post on Tuesday, the group clarified that it “adheres to right-wing conservative political views and traditionalist beliefs.”
The group says that it consists of mostly volunteers, ethnic Russians and Russian citizens living in Ukraine, who have fought for Kyiv since 2014.
The Freedom of Russia Legion is the other group that claims to have been involved, and also portrays itself as Russians who are fighting for Ukraine and against Putin.
The legion says on its website that it was formed last spring “out of the wish of the Russians themselves to fight against Putin’s armed gang in the ranks of the Armed Forces of Ukraine.”
The group calls on Russian soldiers and officers to join them in the fight for “Free Russia.”
It claims to be officially recognized by Ukraine’s army, and fighting “under the leadership of the Ukrainian command.”
The group was banned as a terrorist organization by Russia’s Supreme Court in March.
No particular names of leaders or people affiliated with the Legion are shared on the group’s site. But a man nicknamed “Caesar” has appeared in videos as its de facto spokesperson.
“We are Russians, just like you,” he said in a video on Monday ahead of the raid, as he decried corruption and Putin’s crackdown of civil liberties. “We are coming home,” he said.
The Legion said in March that Russia started a criminal case against “Caesar,” without identifying his real name.
The Legion also has a strong presence on Telegram, with more than a quarter million subscribers, where they call themselves “free citizens of Russia.”
Earlier this month, RVC vowed to fight in tandem with the Legion “despite a different ideological base.” It’s not clear how many fighters there are in either group.
“The Russian Volunteer Corps does seem especially heavily drawn from Russian nationalists and neo-Nazis, while the Legion appears less politically-aligned,” Mark Galeotti, who heads the Russia-focused consultancy Mayak Intelligence, told NBC News.
“They share a desire to see the Putin regime fall and believe a Ukrainian victory is the best way to further this goal,” Galeotti said.
What's their motivation?
Kapustin, the commander of the RVC, told the press near the Russian border Wednesday that his group makes no secret of some of its members’ right-wing views, but he doesn’t think that being called a neo-Nazi is an insult.
“You will never find me waving a flag with a swastika, you will never find me, I don’t know, raising my hand in a Hitler salute,” he said.
Sporting a goatee and all-black military fatigues, Kapustin touted the success of the Belgorod raid and said a wider operation was being planned.
The group says that it fully recognizes Ukraine’s territorial integrity and considers Putin’s war in Ukraine “criminal.” It lists the “overthrow of the ruling regime in Russia” as one of its objectives.
Meanwhile, Alexei Baranovsky, the spokesperson for the political wing of the Legion, told Reuters the incursion was “the first steps in the main objective of overthrowing Putin’s regime through armed force.”
In a post on the group’s Telegram channel on Wednesday the Legion vowed to return soon. “Belgorod, Bryansk, Kursk, Voronezh, Rostov, Moscow — wait for us,” the post said.
The group’s manifesto calls Putin’s regime “dictatorial,” and says they are fighting against him and “for true freedom for every Russian.” It also calls on both ethnic Russians and minorities in the country to rise against the Russian leader.
“It’s clear that Freedom of Russia Legion and Russian Volunteer Corps are both predominantly Russian groups — self-styled ‘partisans’ trying to bring the Putin government down and that they range from the soccer-thug neo-Nazis to the wannabe celebrities and even to some semi-serious political reformers,” said Michael Clarke, visiting professor of war studies at King’s College London.
“They are not ‘liberals’, but rather hard line Russian nationalists — just not of the Putin variety,” Clarke added.
NBC News has reached out for comment to both groups, but they did not immediately respond.
What’s their connection to Ukraine?
The Kremlin does not dispute that some of the fighters involved in the incursion this week could be ethnically Russian, but sees them as “Ukrainian militants, coming from Ukraine,” according to spokesman Dmitry Peskov.
Ukraine has denied it’s connected to actions by either group, framing them as “Russian patriots” who decided to rise up against the Kremlin.
The groups’ extreme views and evidence that the fighters involved in this week’s raid appeared to use American armored vehicles has raised questions for Washington, which has sought to ensure that equipment sent to Kyiv is not used in attacks on Russian soil.
Andriy Yusov, a spokesman for Ukraine’s military intelligence directorate, told NBC News the incursion was carried out “exclusively by citizens of the Russian Federation” who had acted “completely autonomously” and that Ukraine did not coordinate with them.
Baranovsky, of the Legion, said the unit was part of Ukraine’s international brigade, but denied the incursion was coordinated with Ukrainian authorities.
But Kapustin, the RVC commander, said Ukraine “encouraged” the group’s actions in Belgorod, and that they “consult” on their actions with Ukraine’s military, though he said anything they do outside the country’s borders “is our own decision.”
Experts also questioned Kyiv’s narrative that it was a purely Russian affair.
“There’s no question really that the Russian volunteer units are armed and controlled by Ukraine,” Galeotti said.
“Although it suits Kyiv to pretend that this was simply a venture by Russian patriots, it would only have been carried out by GUR’s orders, or at least with its approval,” he added, referring to the intelligence branch of Ukraine’s defense ministry.