The chief of Russia's Wagner mercenary group, Yevgeny Prigozhin, said his men fighting in the flashpoint eastern Ukrainian city of Bakhmut would kill rival soldiers and take no more prisoners.
He was reacting to a Wagner-affiliated Telegram channel's posting of an alleged recording of what it said were two Ukrainians deciding to shoot a Russian prisoner of war.
The channel did not say where the recording came from and there is no way of verifying its authenticity.
"We will kill everyone on the battlefield. Take no more prisoners of war!" Prigozhin said in an audio recording on Sunday.
"We don't know the name of our guy shot by Ukrainians," Prigozhin said, adding that under international law his group was obliged to "take care, treat, not hurt" any prisoners of war.
He said his group did not want to breach international law and would kill all soldiers on the battlefield.
Wagner's presence has been reported in conflict zones including Ukraine, Syria, Libya, Mali, and the Central African Republic, where it has been accused of abuses.
Moscow and Kyiv have repeatedly accused each other of killing prisoners of war since Russian President Vladimir Putin sent troops to invade Ukraine more than a year ago.
Ukraine's President Volodymyr Zelensky in April denounced Russian "beasts" after a video appeared to show the beheading of a Ukrainian prisoner.
Prigozhin denied accusations from a Russian NGO and a Wagner deserter that members of his fighting force were responsible.