A video posted to social media supposedly shows a partisan fighter with a captured Russian vehicle. (Photo courtesy of Twitter)
The groups -- the Russian Volunteer Corps (RVC) and the Freedom of Russia Legion -- have disclosed little about the financing, numbers, or degree of support they receive from Ukraine.
The RVC is better known because in April its fighters carried out a high-profile mission into Russia's Bryansk region bordering Ukraine.
This time, it wrote on Telegram, the two groups "agreed to combine their efforts and go into battle jointly".
The RVC says it has "right-wing-conservative beliefs".
Its founder Denis Kapustin -- also known as Denis Nikitin -- has links to the far-right and football hooliganism, and previously organized mixed martial arts fighting events and ran a clothing brand called White Rex.
Russia has declared him a terrorist.
The RVC posted a video of the incursion showing a ginger-bearded man wearing the unit's logo.
The Agentstvo website identified him as Alexei Levkin, founder of a neo-Nazi website called Wotanjugend and who previously served in Ukraine's Azov battalion.
The Freedom of Russia Legion's political leader is former Russian lawmaker Ilya Ponomaryov, the only MP to vote against the annexation of Crimea and who later moved to Ukraine.
Its logo is a clenched fist. A video posted on its social media shows a man in camouflage saying they took up arms against the "Kremlin dictatorship". Russia has ruled it is a terrorist organization.
Ukraine's rolling war news show on Tuesday featured a fighter from the Legion with the call sign Caesar. Agentstvo wrote that he was around a decade ago linked to a Russian white supremacist organization called Imperial Legion.
The RVC says it is now carrying out sabotage and reconnaissance missions.
A fighter in the Corps with the call sign Fortune told AFP in March that it coordinates with Ukrainian armed forces when on Ukrainian soil but takes on missions to Russia at its own risk.
He said it had "hundreds" of fighters.
The Legion calls itself a "partisan" movement with political and military wings.
Its website says it was founded in spring 2022, aiming to drive Russian troops out of Ukraine and then "build a new free Russia".
There has been little information about its role in the fighting.
It says it carries out direct action to damage or destroy railway tracks and military infrastructure in Russia.
A fighter with the call sign Caesar said in a TV interview that the two groups were carrying out a "peacekeeping operation" in Belgorod.
"In the last days we've proved... we can successfully carry out quite a large-scale operation, the scale of which will increase in time," he said, vowing there would be more acts of armed resistance in Russia.
Ponomaryov told Britain's LBC radio this week that Ukrainians are "helping us in terms of training our forces and providing us with the necessary equipment".
The Legion wrote on Twitter that it has French RT61 heavy mortars.
The RVC posted a video of fighters riding on an armored personnel carrier that it said was a trophy belonging to Russia's FSB security service, which oversees border guards.
Russian military blog Rybar on Telegram, which has over one million subscribers, wrote that the "main aim of the Ukrainian units is to deflect attention, sow panic and discredit Russian authorities".
Wagner mercenary group founder Yevgeny Prigozhin blamed the incident on failures of the Russian state.
"Instead of ensuring the security of the state, some are syphoning off cash, others are acting like fools," he wrote in a press statement.