Wagner, the Russian mercenary group, says it has stopped recruiting prisoners.

The founder of the Kremlin-affiliated Wagner mercenary group, known for its ruthless tactics and harsh discipline, said on Thursday that it would no longer recruit fighters in Russian prisons. That would end a practice that has raised criticism from human rights groups but helped Moscow grind out advances in Ukraine’s east.

The claim by the founder, Yevgeny Prigozhin, was a response to a report from a Russian news website, Live24, that many prison inmates said Wagner had stopped its recruitment process more than a month ago.

Wagner mercenaries have emerged as some of the most capable Russian fighters in Ukraine, helping Moscow grind out slow, brutal advances in particular around the eastern city of Bakhmut. But thousands of convicts poorly trained for combat have been killed, many within days or even hours of arriving at the front, according to Russian rights advocates and Ukrainian officials.

Mr. Prigozhin said in a statement released through his catering company’s press service that recruitment in prisons had stopped “in full,” but that the company is fulfilling its payment and other obligations to inmates who are already serving. Mr. Prigozhin, who visited Russian prisons in person to recruit fighters, did not specify the reasons behind the decision.

It was not possible to independently confirm Mr. Prigozhin’s claim. The British Defense Ministry said last week that the scale of Wagner’s recruitment of prisoners had “probably significantly reduced” since its peak over the summer and fall.

Inmates signing a contract with Wagner, which started recruiting in prisons last summer, committed to six months of fighting in Ukraine in return for their release from detention. As those contracts have expired, Wagner had sought to replenish its forces.

But that has appeared increasingly difficult as news of Wagner’s high casualty rate reached penal colonies.

Mr. Prigozhin, who gained fame as a restaurateur, has become a close associate of President Vladimir V. Putin and parlayed that relationship into a business empire. Since the invasion of Ukraine, he has become an increasingly important political player in Russia as his mercenaries have played a bigger role on the battlefield.

In January, Mr. Prigozhin’s fighters announced they had gained control of the town of Soledar, near Bakhmut, the first significant new territory captured by Russia since July.

Wagner’s advances have caused a public scuffle with the Russian military. Until January, the Russian Defense Ministry refrained from mentioning Wagner in its briefings, which prompted a senior Wagner commander to accuse the ministry of stealing the group’s achievements.

Olga Romanova, head of the Russia Behind Bars prisoners’ rights association, said on Tuesday in an interview with Current Time, a division of Radio Free Liberty, that the Defense Ministry has been trying to recruit prisoners, too.The ministry has not responded to the report.