The attack took place a week after a blast damaged a bridge in Crimea.

RIA news agency cited the ministry as saying 15 other people were wounded in the shooting on Saturday, in Russia's southwestern Belgorod region that borders Ukraine, when two men gunned down a group who had volunteered to take part in the war.

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It said the two assailants — nationals from an unspecified former Soviet republic — had been shot dead. 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.

Some Russian independent media outlets reported that the number of casualties was higher than the official figures.

"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.

The attack took place a week after a blast damaged a bridge in Crimea, the peninsula annexed by Russia from Ukraine in 2014. Earlier in the war, Russia's flagship in the Black Sea blew up and sank.

"During a firearms training session with individuals who voluntarily expressed a desire to participate in the special military operation (against Ukraine), the terrorists opened fire with small arms on the personnel of the unit," RIA cited a defence ministry statement as saying.

Just a day earlier, Putin said Russia should be finished calling up reservists in two weeks, promising an end to a divisive mobilisation that has seen hundreds of thousands of men summoned to fight in Ukraine and huge numbers flee the country.