The Wagner Group's Yevgeny Prigozhin responded to Russian President Vladimir Putin's calling him a "traitor," by saying he will not surrender or turn back.

"Putin was deeply mistaken about the betrayal. We are patriots of our homeland, we fought and are fighting," Prighozin said in an audio message. "No one is going to turn around at the request of the president, the FSB or anyone else, because we do not want the country to live longer in corruption, deception and bureaucracy."

Prigozhin accused Russia's military of targeting a Wagner column with helicopters and jets.

-ABC News' Patrick Reevell

The powerful head of Chechnya, the semi-independent Russian region, Ramzan Kadyrov, said on Saturday he supported President Vladimir Putin.

Kadyrov saiud he fully backs Putin and called Wagner Group leader Yevgeny Prigozhin's actions "treacherous."

Kadyrov has tens of thousands of heavily armed fighters. He said his forces are already moving to "zones of tension."

It raises prospect of Chechen forces fighting with Wagner Group troops.

Kadyrov has previously been friendly with Prigozhin -- his coming out in support of Putin is a boost for Putin, but also raises prospect of serious clashes in Russia.

-ABC News' Patrick Reevell

Russian leaders are "now choosing which side they are on," an advisor to Ukrainian President Volodymyr Zelenskyy said Saturday.

"The next 48 hours will define the new status of Russia," Mykhailo Podolyak said on Twitter. "Either a full-fledged Civil War, or a negotiated Transit of Power, or a temporary respite before the next phase of the downfall of the Putin regime."

He added, "A deafening 'elite' silence is in Russia so far..."

Members of the mercenary Wagner Group have begun moving north "almost certainly aiming to get to Moscow," in what amounts to the "most significant challenge to the Russian state in recent times," the U.K. Ministry of Defense said on Saturday.

"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how the crisis plays out," the ministry said on Twitter.

The Ukrainian military has landed troops on the Russian-held eastern bank of the Dnipro River across from the city of Kherson, according to Russian reports.

Media posted online by pro-Russian accounts suggested small boatloads of Ukrainian soldiers have managed to establish a small beachhead at the foot of the destroyed Antonivskiy Road Bridge that spanned the river before Ukraine brought it down last year.

The size of the Ukrainian force on the bank is unclear, but Russian accounts suggested it was relatively small.

Some Russian accounts posted dramatic video showing fighting on the eastern bank, including what appears to be a Russian armored vehicle firing intensively at Ukrainian soldiers as it recovers Russian wounded.

The video was undated but Russian reports suggested around several dozen Ukrainian troops landed on June 24 and Russian airborne units have been trying to dislodge them since.

Another video shows a small boat carrying perhaps a dozen Ukrainian soldiers landing by the ruined bridge, coming under shell fire.

The Russian military blogger account, Two Majors, reported a small group of Ukrainian soldiers had succeeded in digging in around the bridge. It noted Russian forces had been forced to pull back to a distance from the bank because their positions had been flooded after the Kakhovka dam was blown up earlier this month.

Russian military bloggers said Russian aircraft and artillery were firing on the Ukrainians Monday.

If Ukraine is able to keep hold of its foothold, it will put further pressure on Russia’s forces in the south, already battling to hold back Ukraine’s counteroffensive on the Zaporizhzhia front.

-ABC News' Patrick Reevell