Washington DC - US spy agencies picked up signs days ago that mercenary chief Yevgeny Prigozhin was preparing to rise up against Russia's defense establishment, several media outlets reported on Saturday.

Intelligence officials conducted briefings at the White House, at the Pentagon, and on Capitol Hill about the potential for unrest in nuclear-armed Russia a full day before it unfolded, the Washington Post and New York Times reported.

Spy agencies first began tracking indications that Prigozhin and his Wagner mercenary force intended to move against the Russian military leadership in mid-June, the Post said.

The Times said the information was both solid and alarming by mid-week, leading to the flurry of briefings.

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In an uprising that played out with dizzying speed, Prigozhin's forces moved from their camps in Ukraine into Russia Friday and took over a regional military command in the southern city of Rostov-on-Don, before advancing toward Moscow.

Just as suddenly, the advance was called off on Saturday, and Russian state media said the Wagner troops would return to Ukraine while Prigozhin would flee to neighboring Belarus.

The Kremlin said it would not prosecute Prigozhin or the armed members of the Wagner group.

As US intelligence officials pinned down information that Prigozhin was preparing military action, they grew concerned about chaos in a country with a powerful nuclear arsenal, the Times reported.

US spy agencies believe that Putin himself was informed that Prigozhin, once a close ally, was plotting his rebellion at least a day before it occurred, the Post reported.