Originally Published: 23 JUN 23 22:35 ET

Updated: 23 JUN 23 22:37 ET

By Kevin Liptak, CNN

(CNN) — American officials determined as early as January there was an internal power struggle underway between the private military group Wagner and the Russian government, and have been gathering and closely monitoring intelligence on the volatile dynamic ever since, according to officials.

As Russia’s invasion of Ukraine stalled earlier this year, top American officials said they saw indications of tensions between the Kremlin and the Wagner Group chief Yevgeny Prigozhin. They said they believed those tensions would mount over the coming months.

Officials said their assessments of the situation derived from intelligence, an indication of how seriously the White House and Pentagon took the potential for a power struggle to cause further instability in the ongoing conflict.

Prigozhin has also frequently aired his complaints in public, putting out a host of audio messages and videos in recent months railing against Moscow’s military leadership, particularly Defense Minister Sergei Shoigu and chief of the Russian armed forces Gen. Valery Gerasimov.

Russia’s domestic intelligence service opened a criminal case against Prigozhin after he appeared to cross a red line with the Kremlin on Friday.

Prigozhin, one of Russian President Vladimir Putin’s closest allies, vowed to retaliate against the country’s military leadership after accusing Russian forces of striking a Wagner military camp and killing a “huge amount” of his mercenaries.

Russia’s Ministry of Defense denied the claims, calling it an “informational provocation.”

As of Friday evening, US President Joe Biden had been briefed on the developing situation in Russia, according to National Security Council spokesman Adam Hodge.

In January, a top White House official said Wagner was becoming a “rival power center to the Russian military and other Russian militaries.” Officials suggested at the time that Prigozhin was working to advance his own interests in Ukraine instead of the broader Russian objectives.

The Wagner Group, which the West claimed had recruited prison convicts for fighting in Ukraine, was making decisions based on “what they will generate for Prigozhin, in terms of positive publicity,” John Kirby, National Security Council coordinator for strategic communications, said in January.

Since then, White House and other US national security aides have been highly attuned to what one official said was an “ongoing battle” between Prigozhin and the Russian ministry of defense.

CNN previously reported that Prigozhin had confronted Putin directly last year about his belief that the war in Ukraine was being mismanaged by top generals who were in charge, according to US officials. At the time, US intelligence officials deemed the Prigozhin meeting as significant enough to include in one of Biden’s daily briefings, sources said.

While it was unclear how Putin reacted to that confrontation, US intelligence officials believed at the time it was further evidence that Prigozhin, who is not part of the Russian government, was trying to assert his influence as the US closely watched the power structures inside the Kremlin. For its part, the Kremlin denied that officials had criticized the management of the war.