Russian officials have claimed that Ukrainian military saboteurs launched an attack across the border, wounding eight people in a small town.
Kyiv officials denied any link with the group and blamed the fighting on a revolt by disgruntled Russians against the Kremlin.
Neither version of events could be independently verified in an area that has witnessed sporadic spillover from the almost 15-month war in Ukraine.
The governor of Russia’s Belgorod region, which borders Ukraine, said that a Ukrainian Armed Forces saboteur group entered the town of Graivoron, about five kilometres (three miles) from the border.
Governor Vyacheslav Gladkov said eight people were wounded and most residents have left the area, but the situation remains “tense”.
Three houses and an administrative building were damaged, he said.
In nearby Zamostye village, a projectile hit a pre-school and caused a fire.
One woman was wounded in her hand, Mr Gladkov said.
He also reported that Russian anti-aircraft systems shot down an unmanned aerial vehicle over Belgorod region.
Mr Gladkov said a counterterrorist operation was under way and that authorities were imposing special controls, including personal document checks and stopping the work of companies that use “explosives, radioactive, chemically and biologically hazardous substances”.
Kremlin spokesman Dmitry Peskov said Russian President Vladimir Putin was informed about the alleged saboteur incursion.
Mr Peskov described the action as an attempt by Ukraine to divert attention from the eastern city of Bakhmut, which Moscow claimed to have captured after months of battle but where Kyiv says it is still fighting.
But Ukrainian military intelligence officials did not confirm that Kyiv had deployed saboteurs.
Instead, they claimed that Russian citizens seeking regime change in Moscow were behind the Graivoron incursion.
Ukraine intelligence representative Andrii Cherniak said Russian citizens belonging to murky groups calling themselves the Russian Volunteer Corps and the “Freedom of Russia” Legion were behind the assault.
Ukrainian President Volodymyr Zelensky’s adviser, Mykhailo Podolyak, said on Twitter that Ukraine “has nothing to do with it”.
He suggested an “armed guerrilla movement” was behind the attack.
The Russian Volunteer Corps claimed in a Telegram post it had crossed the border into Russia again, after claiming to have breached the border in early March.
The Russian Volunteer Corps describes itself as “a volunteer formation fighting on Ukraine’s side”.
Little is known about the group, and it is not clear if it has any ties with the Ukrainian military.
The same is true for the “Freedom of Russia” Legion.
The RVC was founded last August and reportedly consists mostly of anti-Putin far-right Russian extremists who have links with Ukrainian far-right groups.
Earlier on Monday, Ukraine’s Zaporizhzhia nuclear power plant, Europe’s largest atomic power station, spent hours operating on emergency diesel generators on Monday after losing its external power supply for the seventh time since Russia’s full-scale invasion of its neighbour, the head of the UN nuclear watchdog said.
“The nuclear safety situation at the plant (is) extremely vulnerable,” Rafael Grossi, head of the International Atomic Energy Agency (IAEA), said in a tweet.
Hours later, national energy company Ukrenergo said on Telegram that it had restored the power line that feeds the plant.
But for Mr Grossi, it was another reminder of what is at stake at the Russian-occupied plant which has seen shelling close by.
“We must agree to protect (the) plant now; this situation cannot continue,” Mr Grossi said, in his latest appeal for the area to be spared from the fighting between Ukrainian and Russian forces.
The plant’s six nuclear reactors, which are protected by a reinforced shelter able to withstand an errant shell or rocket, have been shut down.
But a disruption in the electrical supply could disable cooling systems that are essential for the reactors’ safety even when they are shut down.
Emergency diesel generators, which officials say can keep the plant operational for 10 days, can be unreliable.
Mr Grossi said it was the seventh time the plant had lost its outside power supply since Russia’s full-scale invasion in February 2022.
The Zaporizhzhia nuclear power plant is one of the 10 biggest atomic power stations in the world.
Ukraine’s presidential office said on Monday morning that at least three Ukrainian civilians were killed and 16 others were injured in Russian assaults over the previous 24 hours.
The Ukrainian Air Force reported that four out of 16 Russian missiles and all 20 drones launched against Ukrainian targets were shot down.
Military targets and public infrastructure in Dnipro, Ukraine’s fourth-largest city in the centre of the country, were singled out for Russian attacks, which injured eight people, officials said.
The Dnipro fire department was affected, and 12 houses, shops, and a pre-school were damaged, according to governor Serhii Lysak.