At least 60 Russian troops have been killed after a training area in occupied eastern Ukraine was hit by two missiles, reports say.
Sources familiar with the situation told the BBC that troops had gathered at the site in Donetsk region for the arrival of a senior commander.
Video footage of the incident appeared to show large numbers of dead.
A Russian official confirmed that a strike took place but described the reports as “grossly exaggerated”.
The attack reportedly came hours before Russian President Vladimir Putin met his Defence Minister Sergei Shoigu.
At the meeting, Mr Shoigu claimed Russian successes in several areas of the front line and spoke of the recent capture of the town of Avdiivka, but made no mention of the Donetsk region incident.
Reports say members of the 36th motorized rifle brigade, normally based in the Transbaikal region of Siberia, were waiting for the arrival of Maj-Gen Oleg Moiseyev, commander of the 29th Army of the Eastern military region, at a training area near the village of Trudovske.
A soldier who survived the incident said during a video recording of the aftermath that the brigade’s commanders had made them stand in an open field.
They were reportedly hit by two missiles fired from the US-made HIMARS launch system.
This and other videos and stills show dozens of soldiers apparently lying dead in a field. Estimates, including by those who survived, suggest at least 60 have died.
The BBC is working to verify the footage.
Transbaikal governor Alexander Osipov indirectly confirmed the strike in his Telegram channel, but said that the reports about it were “inaccurate and grossly exaggerated”.
Without giving casualty figures, he said full and accurate information would be provided to the families of all the soldiers involved.
“No-one will be left without help or support,” he added.
There has been no word about the strike as yet from the Ukrainian authorities.
In a separate development, several pro-Russian sources have reported that the military blogger Andrey Morozov, known as Murz, has killed himself.
Morozov, whose Telegram channel has some 100,000 subscribers, wrote in a series of apparently final posts that he had been forced by the military to take down a report about Russian losses in recent battles, including Avdiivka.
He had said about 16,000 troops had been killed or seriously injured in the campaign and 300 pieces of armour destroyed.
The blogger wrote that he had been shut down by propagandists from state TV, but that they were too cowardly to come and kill him.
“Well I’ll do it myself then,” he adds. “I’ll shoot myself if no-one dares to take on this trifling matter.”
The BBC is unable to verify reports of the blogger’s death or how he might have died.
Russia’s military rarely reports casualties, but some pro-Russian military bloggers have regularly done so. Ukraine has also spoken of thousands of Russian troops killed in recent battles.
And BBC Russian, in a joint project with the Mediazona website, recently updated its figures for confirmed deaths in the Russian military based on open sources in the two years since Russia began its invasion of Ukraine.
Altogether, 45,123 are confirmed dead, including 6,614 since October last year. Since that date, there has been a sharp increase in average weekly deaths compared with previous months.
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