Two men gunned down a group of Russian soldiers on a military firing range in Belgorod on Saturday, killing 11 and wounding 15, Moscow authorities said.
The Defence Ministry said the two assailants from an unnamed nation in the Commonwealth of the Independent States, which groups nine ex-Soviet republics, were killed by return fire.
The ministry labelled the incident a "terrorist attack".
"During a firearms training session with individuals who voluntarily expressed a desire to participate in (the war), the terrorists opened fire with small arms on the personnel of the unit," RIA cited a defence ministry statement as saying.
Oleksiy Arestovych, an advisor to Ukrainian President Volodymyr Zelenskyy, said in a YouTube interview that the attackers were from the Central Asian nation of Tajikistan and had opened fire on the others after an argument over religion.
Tajikistan is a predominantly Muslim nation, while a majority of Russians living in western parts of the country, including Moscow, are Eastern Orthodox.
Some Russian independent media outlets reported that the number of casualties was higher than the official figures.
"A terrible event happened on our territory, on the territory of one of the military units," the governor of Belgorod region, Vyacheslav Gladkov, said early on Sunday.
"Many soldiers were killed and wounded ... There are no residents of the Belgorod region among the wounded and killed," Gladkov said in a video post on the Telegram messaging app.
Euronews was not able to independently verify the nationality of the two men or the Kremlin's casualty numbers.
Putin's poorly organised partial mobilisation to blame?
The shooting comes amid a hasty partial mobilisation ordered by President Vladimir Putin to beef up Russian forces in Ukraine — a move that triggered protests and caused hundreds of thousands to flee Russia.
Putin said Friday that over 220,000 reservists had already been called up as part of an effort to recruit 300,000. He promised the mobilisation would be wrapped up in two weeks.
The mobilisation was troubled from the start, with authorities issuing confusing signals about who should be called up for service in a country where almost all men under age 65 are listed as reservists.
Even though the Russian leader declared that only people who had recently served in the military would be subject to the call-up, activists and rights groups reported military conscription offices rounding up people without any army experience — some of whom were also unfit for service for medical reasons — seemingly focusing their efforts on the more remote regions of the country.
Some of the freshly called-up reservists posted videos of themselves being forced to sleep on the floor or even outside and given rusty weapons before being sent to the front lines.
Russian media reports said some of those who were mobilised were sent to combat without receiving proper training and were quickly killed.
Authorities have acknowledged the mobilisation was often poorly organised and promised to improve the situation.