France has said that there was justifiable speculation about the causes of the plane crash that allegedly killed Wagner mercenary leader Yevgeny Prigozhin, after even the US president hinted at the possible involvement of Russian President Vladimir Putin.

 "We don't yet know the circumstances of this crash. We can have some reasonable doubts," government spokesman Olivier Veran told France 2 television.

Veran said that US President Joe Biden's claim that little "happens in Russia that (President Vladimir) Putin is not behind" was "a truth that can be established" as a general rule.

Prigozhin was "the man who did Putin's dirty work. What he has done is inseparable from the policies of Putin, who gave him the responsibility to carry out abuses as the head of Wagner," he said.

"Prigozhin leaves behind him mass graves. He leaves behind him messes across a large part of the globe, I'm thinking of Africa, Ukraine, and Russia itself," Veran added.