The head of Russian mercenary outfit Wagner Tuesday denied the group's involvement in the execution of one of its fighters in Ukraine, after having said the man deserved a "dog's death".

Last week, social media accounts linked to Wagner shared footage of the brutal killing of a man who had reportedly surrendered to Ukrainian forces before being returned to Moscow's forces.

The man, who identified himself as Yevgeny Nuzhin, was shown receiving a fatal blow to the head with a sledgehammer.

Wagner's head Yevgeny Prigozhin -- a businessman close to the Kremlin -- on Sunday praised what he described as "magnificent work", calling the murdered man a "dog".

"A dog should have a dog's death," he said.

In a fresh statement Tuesday, Prigozhin denied his private fighting group had played any role in the execution and instead blamed US secret services without substantiating his accusations.

"It's the practice of US intelligence services, which abduct people, including Russian citizens, across the world," Prigozhin said, calling on Russian prosecutors to open an investigation.

"Wagner employees are distinguished by their exemplary discipline and strict adherence to international standards and globally accepted rules of social behaviour," Prigozhin added.

The Russian rights group, Gulagu.net which advocates for prisoners in Russian detention said that Nuzhin had been in prison in Russia and was recruited by Wagner to fight in Ukraine.

Prigozhin has been accused of personally participating in a recruitment drive by offering contracts at Russian prisons, vowing that those who surrender or are captured would be killed.

The 61-year-old businessman in September disclosed for the first time that he had founded the Wagner group in 2014 to fight in Ukraine and acknowledged its presence in Africa, the Middle East and Latin America.

Several rights groups have brought legal proceedings against Wagner, accusing its members of having in 2017 tortured and murdered a pro-regime fighter who had deserted in Syria.

This month Wagner opened a headquarters in Russia's second city of Saint Petersburg.