US President Joe Biden has been briefed on the developing situation in Russia, as US officials describe the potential Wagner coup as "serious".
Mr Biden is being kept up to date as the Wagner mercenary chief Yevgeny Prigozhin was accused by Moscow of launching an armed rebellion. A White House National Security Council spokesperson said: "We are monitoring the situation and will be consulting with allies and partners on these developments."
This afternoon, Wagner mercenary leader Yevgeny Prigozhin agreed to "stop" the movement of his troops who are advancing on the Russian capital, reports the BBC.
According to a US official, the crisis is "real" but the Pentagon and White House are reportedly waiting to see how it develops. Former CIA chief of Russia operations, Steve Hall, said it was in a "different league" to Prigozhin's previous complaints against the Russian military hierarchy.
"This might look like a well known Putin tactic where he turns people against each other in his own government, but this is in a different league," he said.
"Most worrisome for the Kremlin is this promise: 'We're all going to march'. These 25,000 guys - to Moscow. That sounds very much like a putsch, like a coup.
"That's certainly why the FSB is involved, and there's probably a direct link to Putin there He would have been the one who said to the FSB, you've got to go in and do something. There's got to be a lot of worry in the Kremlin tonight."
Belarusian President Alexander Lukashenko said this afternoon he had held talks with Wagner mercenary group head Yevgeny Prigozhin during which Prigozhin agreed to stop his troops and "de-escalate the situation", Rossiya 24 news channel has repored..
"Prigozhin accepted Lukashenko's proposal to stop the movement of Wagner in Russian territory and on further steps to de-escalate tension," Rossiya 24 said, quoting Lukashenko's press service.
It also said it was proving "possible to find an acceptable variant of de-escalating [the situation] with security guarantees for Wagner PMC's fighters".
Rossiya 24 said the conversation had been agreed with Putin.
Adam Kinzinger, a former congressman who served on the House foreign affairs committee, said those at the Pentagon and White House would be starting to "think through every worst case scenario."
Speaking to CNN he said: "Putin out of the picture generally would be great for the world. But you think about the instability for Russia that would inevitably come from that, even if this whole supposed coup fails.
"There will be instability for Russia, which has nuclear weapons. There's concerns about proliferation if the Russian republic falls apart into even more pieces."
He described it as a "massive blow to the Russian republic" and "a massive blow to their military effort" in Ukraine.
Developments are also being tracked in Kyiv. Its armed forces posted on Twitter simply: "We are watching."
Mykailo Podolyak, one of Ukrainian President Volodymr Zelensky's top advisers, warned "tumultuous times are coming" for Russia.
Prigozhin had humble beginnings, as a former hot dog stand owner who branched into restaurants which drew interest from Putin. His business expanded significantly to catering and providing school lunches.
In 2010, Putin helped open Prigozhin's factory, which was built thanks to generous loans by a state bank. He also organised catering for kremlin events for several years - earning him the nickname 'Putin's chef' - as well as providing catering and utility services to the Russian military.
Prigozhin's Wagner Group has come to play a central role in Putin's projection of Russian influence in areas of conflict around the world. The United States, European Union, United Nations and others say the mercenary force has involved itself in conflicts in countries across Africa in particular.
Wagner fighters allegedly provide security for national leaders or warlords in exchange for lucrative payments, often including a share of gold or other natural resources. US officials say Russia may also be using Wagner's work in Africa to support its war in Ukraine.
Prigozhin's mercenaries have become a major force in the war in Ukraine, fighting as counterparts to the Russian army in battles with Ukrainian forces. It includes Wagner fighters taking Bakmut, the city which has seen some of the bloodiest and longest battles.
According to estimates from the US, almost half of the 20,000 Russian troops killed in Ukraine since December were Wagner fighters in Bakhmut. Soldiers-for-hire included inmates recruited from Russia's prisons.
As his forces fought and died en masse in Ukraine, Prigozhin raged against Russia's military top brass. A video released by his team last month showed Prigozhin standing next to rows of bodies he said were Wagner fighters.
He accused Russia's military of incompetence and of starving his troops of the weapons and ammunition they needed to fight.
"These are someone's fathers and someone's sons," he said. "The scum that doesn't give us ammunition will eat their guts in hell."
As Prigozhin grew more outspoken against Russia's conventional military fighting tactics in Ukraine, he continued to play a seemingly indispensable role in the Russian offensive and seemed to suffer no retaliation from Putin over his criticism of the President's top generals.
