Wounded Wagner Group soldiers are being left on battlefields for hours, Ukrainian intelligence said.
The deaths of thousands of Wagner soldiers do not matter to Russian society, it suggested.
UK intelligence said the mercenary group recruits from prisons, seeing those troops as "expendable."
A Ukrainian military intelligence report obtained by CNN claims that injured fighters from Russia's Wagner mercenary group are being left on the battlefield for hours without treatment, with colleagues forbidden from helping them.
The report, dated from December, suggests that those making military decisions view the mercenary force, which includes recruits from Russian prisons, as disposable.
"The deaths of thousands of Wagner soldiers do not matter to Russian society," the report said, according to CNN.
Wagner soldiers are also being killed if they withdraw from an area without permission, the document said.
"Assault groups do not withdraw without a command … Unauthorized withdrawal of a team or without being wounded is punishable by execution on the spot."
"Assault infantry is not allowed to carry the wounded off the battlefield on their own, as their main task is to continue the assault until the goal is achieved," it added. "If the assault fails, retreat is also allowed only at night."
At the same time, Russian prisoners who were sent to fight for the group in Ukraine say they have seen colleagues publicly executed if they deserted or did not follow orders.
The Wagner Group, which the US says is responsible for "atrocities and human rights abuses' in Ukraine, has close links to the Kremlin.
The group has become more powerful as Russia's armed forces have struggled to make progress in the war in Ukraine.
However, this had led to rising tension between its commanders and Russia's traditional military command.
The group has around 10,000 mercenaries and 40,000 former prisoners deployed in Ukraine, Insider's Stephanie Stacey reported.
Oleksandr Danylyuk, Ukraine's former national security advisor who now helps the country's military planning, told Politico in December that the Wagner Group was recruiting prisoners and sending them to the front lines where they had "nothing to lose," and that large numbers of them were being killed.
But the Ukrainian intelligence document seen by CNN warned that the Wagner Group was still effective and posed a bigger threat to Ukraine than Russia's army, despite its mounting casualities.
Experts have previously said that the Russian army's own soldiers were also treated as little more than cannon fodder.
Putin called up 300,000 conscripts to fight in Ukraine in September 2022. A month later, some had already been sent to Ukraine, killed, and returned in body bags.
William Alberque, who runs the arms control program at the International Institute for Strategic Studies, told Insider that Putin appeared to be using soldiers as disposable entities in an effort to slow down Ukraine's military advance.
"He's trying to staunch the bleeding, as it were, by throwing bodies at the frontline and slow the Ukrainian advances down – stabilize the battlefront in blood."
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