Two volunteer soldiers fired at other troops at a Russian military firing range near Ukraine, killing 11 and wounding 15 others, before getting killed, the Russian Defense Ministry said.
The ministry said in a statement that the shooting took place in the Belgorod region in southwestern Russia that borders Ukraine.
It said the two volunteers from an unnamed ex-Soviet nation fired on other soldiers during target practice and were killed by return fire.
The ministry called the incident a terrorist attack.
The shooting comes amid a hasty 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.