Russia will not investigate the plane crash that killed Yevgeny Prigozhin under international rules, according to Brazil's aircraft investigation authority.
A Brazilian-made Embraer jet crashed north of Moscow last week, killing the mercenary leader, two of his top lieutenants, four bodyguards and three crew.
Brazil's Centre for Research and Prevention of Aeronautical Accidents (CENIPA) said it would join a Russian-led investigation if the probe was held under international rules.
But Russia is refusing such terms "at the moment", the centre said.
"They are not obliged, only recommended to do that," CENIPA head Air Brigadier Marcelo Moreno said.
"But if they say they'll open the investigation and invite Brazil we will participate from afar."
US aviation safety consultant and former investigator John Cox said an internal Russian investigation would always be questioned without the participation of Brazil, the country where the plane was manufactured.
"I think it's very sad," Cox said. "I think it hurts the transparency of the Russian investigation."
But the United Nations International Civil Aviation Organisation said the flight from Moscow to St Petersburg was domestic, so it is not subject to international rules.
In air crash investigations, experts work to improve aviation safety without assigning blame, but probes are often tainted by political interests.
CENIPA and Embraer said they wanted to prevent future accidents but faced challenges in getting information from the investigation due to sanctions on Russia and Moscow's reluctance to allow outside scrutiny.
The Kremlin denies any involvement in the plane crash.
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