Kyiv, Ukraine - The reported death of Wagner mercenary chief Yevgeny Prigozhin was predictable following his June mutiny against the Russian army leadership, a top adviser to Ukrainian President Volodymyr Zelensky said Wednesday night.

"Prigozhin signed his own death warrant the moment he stopped 200 kilometers [125 miles] from Moscow," Ukrainian presidential adviser Mykhailo Podolyak told the German tabloid Bild.

Prigozhin was killed in a private jet crash near Moscow on Wednesday evening, exactly two months after his failed uprising against his country's military leadership.

The Grey Zone Telegram channel, which is affiliated with Prigozhin's Wagner mercenary army, said the plane was deliberately shot down.

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The channel initially suggested that Prigozhin could have survived, before saying a short time later that he had died.

There has been no official statement from Russian government officials on Prigozhin's fate.

The incident, which remained shrouded in mystery in the early hours of Thursday, ignited speculation about exactly how the outspoken critic of top Russian defense officials met his fate.

Podolyak said that Prigozhin's uprising in June "really frightened" Russian President Vladimir Putin and predictably led to consequences because "Putin doesn't forgive anyone for making him afraid."

Should the theory be confirmed that the crash of the plane with Prigozhin on board was due to a murder plot, it would be an "ostentatious liquidation" and "a direct signal to the elites ... that the brutal murders of 'own people' in Russia are beginning."

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In Podolyak's view, Moscow had also sent a signal to its own army "that there really are no heroes there and that any disloyalty will be punished by death."

US President Joe Biden, meanwhile, also took the news in his stride.

"I don't know for a fact what happened but I'm not surprised," he told reporters in California, where he is vacationing.

Asked if Putin was behind it, Biden said: "There's not much that happens in Russia that Putin's not behind. But I don't know enough to know the answer. I've been working out for the last hour and a half."