At least 11 people were killed and 15 others injured at a Russian military training ground when two attackers opened fire on a group of volunteers seeking to fight in Ukraine, Russia's RIA news agency said.
It said two of the attackers - an unnamed citizen of the former Soviet republic - had been shot dead.
Several independent Russian media reported that the toll was higher than the official figure.
"During firearms training sessions with individuals who voluntarily expressed a desire to participate in special military operations [against Ukraine] the terrorists opened fire with small arms at the personnel of the unit," the RIA quoted the Ministry of Defense as saying.
"As a result of the shooting, 11 people were seriously injured. Another 15 people with injuries of varying severity were taken to medical facilities," he said. No civilians were killed in the attack but many soldiers were killed or injured, the governor of the Belgorod region Vyacheslav Gladkov said on Sunday morning.
"A terrible event occurred on our territory, on the territory of one of the military units," Gladkov said in a video post on the messaging app Telegram.
"Many soldiers were killed and injured. There were no residents of the Belgorod region among the wounded and killed."
Gladkov did not say how many soldiers died.
Oleksiy Arestovych, an adviser to Ukrainian President Volodymyr Zelenskyy, said in a YouTube interview that the attackers were from the Central Asian country of Tajikistan, and had opened fire after arguments over religion. Tajikistan is a Muslim-majority country, while about half of Russians adhere to various branches of Christianity. The Russian ministry said the attackers were from a country in the Commonwealth of Independent States, which groups nine former Soviet republics, including Tajikistan.
Reuters could not immediately confirm comments by Arestovych, a prominent commentator on the war, or independently verify the casualty toll and other details of the incident. Russia's independent news site SOTA Vision said the attack took place in the small town of Soloti, near the Ukrainian border and about 105 km southeast of Belgorod. Authorities in Belgorod have repeatedly accused Ukraine of attacking targets in the city, including the power grid and fuel and ammunition depots.
Ukraine has not yet claimed responsibility. The deadly incident is just the latest in a series of major setbacks for Moscow's forces since its invasion of Ukraine.
Ukrainian energy facilities attacked
Meanwhile, Russian missile strikes have damaged a key energy facility in the Ukrainian capital region, the country's network operator said. Citing security, Ukrainian officials did not identify the site, one of many infrastructure targets the Russian military is trying to destroy after a truck bomb explosion on October 8 damaged a bridge linking Russia with annexed Crimea.
Kyiv Regional Governor Oleksiy Kuleba said there were no casualties from the strike, and electricity company Ukrenergo said repair crews were working to restore service, but warned residents of the possibility of further blackouts.
Kyrylo Tymoshenko, deputy head of the Ukrainian presidential office, urged Kyiv residents and people in three neighboring regions to reduce energy consumption during the evening hours with peak demand.
Analysts suggest the Russian military is trying to cut off electricity and water in populated areas of Ukraine while also defending against Ukrainian counterattacks on occupied territories.
"Putin may hope that by increasing the misery of the Ukrainian people, President [Volodymyr] Zelenskyy may be more inclined to negotiate a settlement that allows Russia to retain some stolen territory in the east or Crimea," said Ian Williams, a fellow at the Center for Strategic and International Studies, a Washington-based policy organization.
"A glimpse of history shows that strategic bombing of civilians is an ineffective means of achieving political goals," he said. The area of southern Ukraine that Vladimir Putin illegally designated as Russian territory last month remained the focus of fighting on Saturday.
Ukrainian troops tried to advance south along the banks of the Dnieper towards the regional capital Kherson, but were unsuccessful, according to Kirill Stremousov, deputy head of regional administration stationed in Moscow.
"The line of defense is functioning, and the situation remains under the complete control of the Russian army," he wrote on his messaging app channel.
Stremousov warned residents that they could flee to Crimea and cities in southwestern Russia as Ukrainian troops tried to advance towards Kherson. Russian fuel depot peeled off.
A fuel depot in Russia's Belgorod region, which borders Ukraine, caught fire after Saturday's shooting, its governor said, without elaborating on the origin of the shooting. Russia's border regions including Belgorod have accused Ukraine of attacking targets including power lines and fuel warehouses since Moscow sent its armed forces to Ukraine on February 24. There was no immediate comment from Kiev.
"We have another shooting. One of the bullets hit an oil depot in the Belgorod district," Belgorod region governor Vyacheslav Gladkov said on social media, posting images of flames and black smoke rising into the air.
"Emergency services are already fighting the fire. There is no danger of [the fire] spreading," he said.
Local emergency services said one in ten tanks with remaining diesel fuel had been burned, the TASS news agency reported. Mr Gladkov later said that the fire had been put out. On Thursday, Russian officials said Ukrainian shells had destroyed an ammunition depot and also hit a school, apartment block and an electrical substation in the Belgorod region.
Gladkov said the customs checkpoint had also been shelled for several days in a row, and 14 shells had landed there on Saturday, but caused no injuries.