Two men opened fire on volunteers at a Russian military firing range near Ukraine, killing 11 people and wounding 15 others, the Russian Defense Ministry said.
The shooting reportedly took place in the Belgorod region in the southwest Russiawhich borders Ukraine.
The Moscow Defense Ministry said the two men from an unnamed former Soviet nation opened fire on other soldiers during gunnery training, only to be killed by return fire.
The ministry called the incident a terrorist attack.
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Belgorod Governor Vyacheslav Gladkov said in a social media post on Telegram: “A terrible event has happened on our territory, on the territory of one of the military units.
“Many soldiers were killed and wounded… there are no residents of the Belgorod region among the wounded and killed.”
The shooting comes after the president Vladimir Poutine ordered a rapid mobilization to reinforce Russian forces in Ukraine.
Earlier, the Belarusian Defense Ministry released images of Russian troops arriving in the country as part of a “regional grouping”.
Footage showed an orchestra playing and female troupes offering food to Russian soldiers.
Meanwhile, the UK Ministry of Defense said many reserve troops called up by Russia were likely to buy their own body armor.
In his Saturday morning briefing, he said the modern 6B45 vest – which is supposed to be standard issue – sells online for 40,000 rubles, up from around 12,000 rubles in April.
Some 300,000 reservists are being called up, Russia says, as it seeks to stem a wave of successful Ukrainian counterattacks. However, the MoD said their equipment “almost certainly falls short of the already poor supply of previously deployed troops.”
The substandard kit adds to expert assessments that many reservists will be ill-prepared and poorly trained for frontline warfare.
President Putin said on Friday the call would be over in two weeks.
A shocking number of Russian corpses, a grim illustration of Ukrainian success
Elsewhere, the US has said it will send more munitions and military vehicles to Ukraine as part of a new $725m (£639.7m) package.
This will bring total US security aid since Russia invaded Ukraine in February to more than $17.5bn (£15.6bn).
The latest package includes high-velocity anti-radiation missiles (HARMs) and precision-guided artillery, as well as medical supplies, the US Department of Defense said in a statement.
It is hoped that this support will bolster Ukraine’s ability to fend off Russia in the counter-offensive that has yielded significant territorial gains in recent weeks.