A former US ambassador to Russia has suggested that part of the Russian military and security services may have supported the rebellion by the private military firm Wagner Group.

It is believed that the Wagner force, led by Yevgeny Prigozhin, took over a Russian military hub in the southern state of Rostov with no resistance.

John Sullivan, who is also a former deputy secretary of state, told NHK on Wednesday that it's "difficult to understand" what happened "if there was no support for Prigozhin within the Russian military and security services."

He also commented on US media reports that the deputy commander of Russia's military operations in Ukraine, Sergei Surovikin, knew about Prigozhin's plans in advance.

Surovikin is said to have been close to Prigozhin.

Sullivan said it's difficult to explain how they took control of military facilities without facing resistance "if there wasn't some prior knowledge by Russian military leaders."

The former ambassador also thinks that Russian President Vladimir Putin has lost some of his strength.

Sullivan said Prigozhin was allowed to go to Belarus because Putin "wanted to avoid chaos and bloodshed" and "had to strike a deal with a person that he labeled a traitor who had stabbed Russia in the back."

Sullivan pointed out that Putin "wasn't strong enough to put down that revolt, arrest Prigozhin or eliminate him."

Asked about the possibility that Putin may lose power, Sullivan said he "wouldn't predict it, at least right away."

But he said it's more likely today than it was a week ago because "this mutiny by Prigozhin has revealed actual weakness in President Putin."