Two armed groups, the Russian Volunteer Corps (RVC) and Freedom of Russia Legion, say they have carried out attacks in Russia's western Belgorod region in recent weeks.
Kyiv has denied any role and said the attackers are Russian citizens.
Here is what we know about the groups.
RUSSIAN VOLUNTEER CORPS
The RVC was founded by a far-right Russian national last August and comprises Russians who have been fighting in and for Ukraine against their own country.
It said on Sunday it was also behind an attack in the Belgorod region in which several soldiers were captured and has previously said it carried out other military incursions including in March and April.
The Ukrainian military intelligence agency says the RVC is an independent underground group inside Russia that also has a unit in the Ukrainian Foreign Legion. The Foreign Legion says it has nothing to do with the RVC.
Denis Kapustin, who described himself as the RVC commander, said the group would carry out more raids when he spoke to reporters on the Ukrainian side of the border on May 24.
The reporters asked him repeatedly about Western media reports that his militia had used U.S. military equipment that was meant to help Ukraine defend itself against Russia's invasion, but he declined to answer directly.
The U.S.-based Anti-Defamation League has described Kapustin as "a Russian neo-Nazi who lived in Germany for many years".
"I have my set of views, it's a patriotic set of views, it's a traditionalist set of views, it's a right-wing set of views," he said. "You know, you'll never find me waving a flag with a swastika, you'll never find me raising my hand in a Hitler sign."
FREEDOM OF RUSSIA LEGION
The Freedom of Russia Legion says it was formed in spring 2022 "out of the wish of Russians to fight in the ranks of the Armed Forces of Ukraine against Putin's armed gang".
The group says it cooperates with the Ukrainian armed forces and operates under Ukrainian command. It said it was responsible for attacking Belgorod last month and that it has been fighting in eastern Ukraine.
Ukrainian military intelligence has said the attacks in Belgorod have involved only Russian citizens and are aimed at creating a "security zone" to protect Ukrainian civilians.
(Reporting by Tom Balmforth and Max Hunder; editing by Timothy Heritage)
