The United States said Sunday that the brief rebellion of mercenary leader Yevgeny Prigozhin against Russia’s military leadership shows “very serious cracks” in the two-decade rule of President Vladimir Putin and “questions the very premise” of his 16-month war against Ukraine.

U.S. Secretary of State Antony Blinken told ABC’s “This Week” show, that the war “has been a devastating strategic failure for Putin across virtually every front – economic, military, geopolitical standing.”

“We see cracks emerging,” Blinken said of Prigozhin’s protest targeting Putin. “Where they go – if anywhere – when they get there, very hard to say. I don’t want to speculate on it. But I don’t think we’ve seen the final act.”

The top U.S. diplomat said the aftermath of Prigozhin’s Wagner Group advance Saturday on Moscow before an abrupt retreat well short of reaching the Russian capital is “still a moving picture,” with the outcome uncertain.

Putin called Prigozhin, a longtime ally whose troops had been fighting alongside Russian forces in Ukraine, a traitor for turning against his authoritarian regime. Then the Russian leader said Prigozhin would not be prosecuted and allowed him to go to neighboring Belarus, a Russian ally, under a deal negotiated by Belarusian leader Alexander Lukashenko.

But it was not clear whether Prigozhin had arrived there and whether he had a contingent of mercenaries with him.

Russia said Prigozhin’s mercenaries who had not joined the short-lived rebellion would be allowed to sign contracts to fight with the Russian army in Ukraine, but Blinken said it was not clear what will happen now to the mercenaries who followed Prigozhin into Russia and headed toward Moscow before Prigozhin called off his troop advance.

“The short answer is we don’t know,” Blinken said.

Asked what happens to Prigozhin in Belarus, aside from his amnesty, Blinken acknowledged, “We simply don’t know.”

Blinken said U.S. President Joe Biden spoke with allied Western leaders from his Camp David retreat outside Washington and conferred with U.S. national security officials but had not called Putin.

Blinken said the “direct challenge to Putin’s authority” showed that Moscow’s war in Ukraine has “been pursued under false pretenses, that NATO was a threat to Russia.” But Prigozhin’s frequent complaints about Moscow’s defense officials centered on his claim that they were not supplying his mercenaries with enough arms to fight in Ukraine.

Many of the Wagner Group forces were convicted criminals that Prigozhin enticed to the war front to fight for six months in exchange for release from their prison terms, if they survived the war. But they were poorly trained and thousands of them were quickly killed on the front lines.

Blinken said the turmoil created in Russia by the Prigozhin protest “may help the Ukrainians on the battle front.”

But David Petraeus, a retired U.S. Army general and former Central Intelligence Agency chief, told CNN’s “State of the Union” show that “unfortunately it has not had a significant effect on the front lines.” But he, too, said the Prigozhin protest leaves Putin “more vulnerable than he’s ever been in two decades.”

Like Blinken, however, Petraeus said there are “many, many unknowns” remaining in determining the overall effect of the Putin-Prigozhin showdown.

The Kremlin said Saturday that criminal charges against Prigozhin for mounting the armed rebellion will be dropped, even though many Russians have been jailed for their public comments or street protests against Putin’s war.

Lukashenko negotiated the deal with the Wagner Group chief, his office said, with Putin’s approval. Lukashenko said he has known Prigozhin personally for 20 years.

The negotiations also guaranteed that Wagner fighters will not be prosecuted, Kremlin spokesman Dmitry Peskov told reporters. “We have always respected their heroic deeds at the front,” he said, adding that Moscow was grateful to Lukashenko for his role in de-escalating the crisis.

Those fighters who did not participate in the rebellion, Peskov said, would be offered contracts with the defense ministry, which has been seeking to bring all autonomous volunteer forces under its control by July 1.

Asked if there would be any personnel changes in the Russian defense ministry as part of the deal, Peskov said, “These matters are the sole prerogative and within the competence of the Supreme Commander-in-Chief [Putin] in accordance with the constitution of the Russian Federation. Therefore, it is unlikely that these topics could have been discussed in the course of the above-mentioned contacts.”

The Russian spokesman did not disclose whether there were any concessions made to persuade Prigozhin to withdraw all his forces, other than the guarantees for his safety — something he said Putin had given his word on — and for the safety of Prigozhin’s men.

He called the events that unfolded since Friday “tragic.”

Earlier Saturday, Prigozhin and his fighters got within about 200 kilometers of Moscow before he ordered his men to halt their advance, turn their convoy around and return to their bases in Ukraine to avoid bloodshed.

Prigozhin didn’t say whether the Kremlin has responded to his demand to oust Defense Minister Sergei Shoigu or what, if anything, Lukashenko had promised him in their negotiations.

Moscow had braced for the arrival of the private army led by the rebellious commander. And Putin had said Prigozhin would face harsh consequences.

By Saturday evening, the Russian mercenary fighters were seen pulling out of Rostov-on-Don, a city of more than one million people near the border with Ukraine that they had taken control of a day earlier.

In Rostov, which serves as the main rear logistical hub for Russia’s entire invasion force, residents had milled about, recording on mobile phones, as Wagner Group mercenary fighters in armored vehicles and battle tanks took up positions, the Reuters news agency reported.

Some information for this story came from The Associated Press, Agence France-Presse and Reuters.