The head of the Wagner mercenary group, which in June attempted to topple Russia's military leadership, was on board a plane that crashed on Wednesday, with all passengers killed, Russian officials said.
The Kremlin and the defence ministry were yet to react.
Yevgeny Prigozhin's short-lived rebellion was seen as the biggest challenge to Russian President Vladimir Putin's authority since he came to power.
Since then, uncertainty has surrounded the fate of Wagner and its controversial chief.
Russia's ministry for emergency situations on Wednesday announced the crash of a private plane travelling between Moscow and Saint Petersburg.
According to preliminary information, all 10 people on board died, including three crew members, the ministry said.
Russia's aviation agency later said the Wagner chief was on board the plane.
"According to the airline, the following passengers were on board the Embraer-135 (EBM-135BJ) aircraft:... Prigozhin, Yevgeny," said Rosaviatsia, which also listed Dmitry Utkin, a shadowy figure who managed Wagner's operations and allegedly served in Russian military intelligence.
Telegram channels linked to Wagner posted footage -- that AFP could not independently confirm -- showing the wreckage of the plane burning in a field.
Russian law enforcement officials were standing guard at the crash site near the village of Kuzhenkino in the Tver region early on Thursday, AFP images showed.
In Saint Petersburg, people laid flowers and patches bearing the Wagner skull logo at a makeshift memorial outside the private mercenary group's headquarters, AFP journalists said.
"Guys, we just have no words right now," said a masked man and alleged members of Wagner at the site.
"Let's support Yevgeny Viktorovich (Prigozhin) and all our commanders. We need your support now."
- 'Not surprised' -
Rosaviatsia said it set up a special commission to investigate the crash of the aircraft belonging to MNT-Aero.
Russia's Investigative Committee, which probes serious crimes, said it opened an investigation into the crash.
The bodies of eight people have been found so far at the site of the crash, RIA Novosti said citing the emergency services.
Putin was meanwhile giving a speech for the 80th anniversary of the Kursk battle in World War II.
He did not mention the crash and hailed "all our soldiers who are fighting bravely and resolutely" in the special military operation in Ukraine.
But Kyiv and Washington reacted swiftly to initial reports of the crash.
"I don't know for a fact what happened, but I'm not surprised," United States President Joe Biden said.
"There's not much that happens in Russia that (President) Putin's not behind. But I don't know enough to know the answer."
Ukrainian presidential aide Mykhaylo Podolyak said on social media that the plane crash was "a signal from Putin to Russia's elites ahead of the 2024 elections. 'Beware! Disloyalty equals death'."
Svetlana Tikhanovskaya, the exiled leader of the opposition of Belarus -- where some Wagner fighters moved after their short-lived mutiny in Russia -- said Prigozhin would not be missed in her country.
"He was a murderer and should be remembered as such," she said on social media.
Prigozhin came into the spotlight during Russia's offensive in Ukraine, launched on February 24, 2022, having previously operated in the shadows.
He spearheaded the capture of several Ukrainian towns including Bakhmut, and harshly criticised Russia's conventional military leadership.
- Uncertainty after rebellion -
But Prigozhin was locked in a bitter months-long power struggle with Russia's defence ministry, which he accused of trying to "steal" Wagner's victories.
Tensions degenerated into a short-lived rebellion on June 23 and 24.
Thousands of mercenaries took up weapons and marched from southern Russia towards Moscow aiming to topple the country's military leaders.
The mutiny ended with a deal under which Prigozhin was expected to move to neighbouring Belarus with some of his men, where they began training the ex-Soviet country's special forces.
But the fate of Prigozhin remained unclear: he seemed to enjoy a certain amount of freedom and took part in a meeting at the Kremlin where he refused to cede command of his mercenary group.
Still, he mostly remained out of the public eye.
His Telegram channel -- where he usually communicated -- has been inactive since the end of June.
Wagner-linked Telegram channels instead purportedly relayed rare messages.
On Monday, video footage circulated showing Prigozhin apparently in Africa, which he vowed to make "freer".
The mercenary group maintains a strong military presence on the continent, where it has partnered with several nations, including Mali and the Central African Republic.
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