The leader of a Russian group involved in a border raid has been described by watchdogs as a neo-Nazi.

One of the anti-Kremlin groups responsible for an armed incursion into Russia this week, the Russian Volunteer Corps, is led by a far-right extremist described by German officials and humanitarian groups as: including the Anti-Defamation Leagueas a neo-Nazi.

The Volunteer Corps, made up of Russians opposed to Vladimir V. Putin’s war, has no public ties whatsoever with the Ukrainian army. But the group’s claims of fighting for Ukraine’s cause pose an uneasy situation for the Kiev government. President Vladimir V. Putin of Russia has falsely claimed that his country is fighting the Nazis as a pretext for invading his country, a consistent theme of Kremlin propaganda.

The corps commander – Denis Kapustin, who has long gone by the alias Denis Nikitin but usually goes by his military call sign White Rex – is a Russian citizen who moved to Germany in the early 2000s. He associated with a group of violent football fans and later became, according to officials in the German state of North Rhine-Westphalia, “one of the most influential activists” in a neo-Nazi splinter of the mixed martial arts scene.

He has been blocked of entering Europe’s visa-free Schengen zone with 27 countries.

The Volunteer Corps, known by its Russian initials RDK, also claimed credit for two incidents in the Russian border region of Bryansk in March and April. Ukrainian authorities have publicly denied any role in the fighting on the Russian side of the border.

The Russian Volunteer Corps was one of two groups of Russian fighters to carry out a cross-border attack on Monday in the Belgorod region of southern Russia, attacking Russian troops during two days of clashes. The purpose of the raids, the groups say, is to force Russia to redeploy soldiers from Ukraine’s occupied territories to defend its borders as Ukraine prepares for a counter-offensive.

The second group was the Free Russia Legion, which operates under the umbrella of the Ukrainian International Legion, a force that includes American and British volunteers, as well as Belarusians, Georgians and others. It is overseen by the Ukrainian Armed Forces and commanded by Ukrainian officers. Several hundred Russian fighters have been deployed to the front lines in eastern Ukraine, officials said.

At a joint press conference with the Free Russia Legion on Wednesday, Mr. Kapustin said his group was not under the control of the Ukrainian army, but that the army had supported his fighters with information, gasoline, food and medical supplies, along with the evacuation of wounded personnel. That claim could not be independently verified.

Andriy Chernyak, a representative of the Ukrainian military intelligence service, said he had no information about possible material support that the Ukrainian army may have provided to members of the RDK, but said that “Ukraine certainly supports all those who are ready to face the Putin regime.”

“People came to Ukraine and said they want to help us fight Putin’s regime, so of course we let them in, like many other people from abroad,” Chernyak said.

Ukraine has called the raids an “internal Russian crisis” as the members of the group are Russians themselves, and the episode plays on a Ukrainian military goal of forcing Russia to redeploy troops from the front lines to defend its borders.

Michael Colborne, a Bellingcat researcher who reports on the international far right, said he was hesitant to even call the Russian Volunteer Corps a military unit.

“They are largely a far-right group of neo-Nazi exiles who are making these incursions into Russian-occupied territory and seem much more concerned about creating social media content than anything else,” Mr Colborne said.

Some of the other members of the Russian Volunteer Corps photographed during the border raids have also publicly espoused neo-Nazi views. A man, Aleksandr Skachkov, was arrested in 2020 by Ukrainian security services for selling a Russian translation of the white supremacist manifesto of the gunman in Christchurch, New Zealand, who killed 51 mosque worshipers in 2019.

Another, Aleksei Levkin, who filmed a selfie video wearing the RDK badge, is a founder of a group called Wotanjugend which started in Russia but later moved to Ukraine. Mr. Levkin also hosts a National Socialist black metal festival, which started in Moscow in 2012 but was held in Kiev from 2014 to 2019.

Photos posted online earlier this week by the fighters of members of the volunteer corps posing in front of captured Russian equipment showed some of the fighters wearing Nazi-style patches and gear. One patch shows a hooded member of the Ku Klux Klan and another shows a black sun, a symbol of strong ties to Nazi Germany.

Mr Colborne said the images of Mr Kapustin and his fighters could damage Ukraine’s defenses by making allies wary that they might support far-right armed groups.

“I’m afraid something like that could backfire on Ukraine, because these are not ambiguous people,” he said. “These are not unknown people and they are not helping Ukraine in any practical way.”

Thomas Gibbons Neff contributed reporting from London and Oleg Matsnev from Berlin.