A US think tank says the leader of the Wagner mercenary group, Yevgeny Prigozhin, was likely assassinated at the order of Russian President Vladimir Putin.

The Institute for the Study of War noted that the Kremlin and the Russian defense ministry had been destroying the Wagner group and weakening Prigozhin's authority since the armed rebellion in June.

It said Prigozhin's assassination was likely the final step to eliminate Wagner as an independent organization.

The institute also said, "Putin almost certainly ordered the Russian military command to shoot down Prigozhin's plane."

It added that Russian defense minister Sergei Shoigu and Chief of the General Staff Valery Gerasimov "would be extremely unlikely to execute Prigozhin without Putin's order."