The head of the Wagner mercenary group said Saturday he had crossed into Russia with his forces to topple Moscow's military leadership, saying he and his 25,000 fighters were "ready to die."

Yevgeny Prigozhin, 62, said his forces, who have spearheaded much of Russia's offensive in Ukraine, had entered the southern Russian city of Rostov-on-Don and controlled its military sites.

"We are inside the (army) headquarters, it is 7:30 am (0430 GMT)," Prigozhin said in a video on Telegram. "Military sites in Rostov, including an aerodrome, are under control," he added.

Prigozhin said planes taking part in the Ukraine offensive "are leaving as normal" from the airfield, and called on Russians not to believe what they were being told on state television.

"A huge amount of territory is lost. Soldiers have been killed three, four times more than it says in documents shown to the top (leadership)."

Videos and pictures posted online, including by the TASS state-run news agency, showed armed men surrounding administrative buildings in Rostov and tanks deployed in the city centre. It was not clear who the armed men were.

President Vladimir Putin was due to make a televised address shortly, Kremlin spokesman Dmitry Peskov told Russian news agencies.

Russian authorities earlier said security had been tightened in several regions, and the mayor of Moscow announced that "anti-terrorist" measures were being taken in the capital.

The FSB security service accused Prigozhin of attempting to launch a "civil conflict" and urged Wagner fighters to detain him.

"All of us are ready to die. All 25,000, and then another 25,000," Prigozhin, said in an earlier audio message, after accusing the Russian top brass of launching strikes against his men.

"We are dying for the Russian people."

The Wagner chief said his forces had also shot down a Russian military helicopter.

"We will destroy everything that stands in our way," Prigozhin said earlier, in the most audacious challenge to Putin since the start of the offensive in Ukraine last year.

In Moscow, critical facilities were "under reinforced protection," TASS reported, citing a law enforcement source.

Prosecutor General Igor Krasnov had informed Putin of "the initiation of a criminal case in connection with an attempt to organise an armed rebellion," Kremlin spokesman Peskov said.

Missile strikes

The extraordinary developments came after Prigozhin accused Moscow of targeting his forces with missile strikes that he said killed "a huge number of our fighters."

"The council of commanders of PMC Wagner has made a decision -- the evil that the military leadership of the country brings must be stopped," he said in a series of furious audio messages released by his spokespeople.

He warned Russians against resisting his forces and called on them to join him.

"We need to put an end to this mess," he said, adding, "this is not a military coup, but a march of justice."

In a statement, the FSB said: "Prigozhin's statements and actions are in fact a call to start an armed civil conflict on the territory of the Russian Federation and a stab in the back to Russian servicemen fighting pro-fascist Ukrainian forces."

While Prigozhin's outfit has been at the forefront of much of Russia's offensive in Ukraine, he has in recent months engaged in a bitter feud with Moscow's military leadership and has repeatedly blamed Defence Minister Sergei Shoigu and Valery Gerasimov, chief of the general staff, for his fighters' deaths.

'Urge you to stop'

The Russian defence ministry denied Prigozhin's claims of an attack on his forces, saying the statements "do not correspond to reality."

It later said Ukrainian troops were taking advantage of the infighting to ready an assault near the eastern hotspot of Bakhmut.

A prominent Russian general urged Prigozhin to call off efforts to remove the defence ministry leadership.

"I urge you to stop," Sergei Surovikin, commander of Russia's aerospace forces, said in a highly unusual video address.

"Before it is too late, it is necessary... to obey the will and order of the popularly elected president of the Russian Federation."

Anti-Kremlin figure Mikhail Khodorkovsky, however, urged Russians to support Prigozhin, saying it was acceptable to back "even the devil" in taking on the Kremlin.

Washington-based think tank the Institute for the Study of War said the Wagner chief's attempt to force a leadership change in the defence ministry "is unlikely to succeed" given that Surovikin had denounced his call for rebellion.

However, it said Wagner's attempts to take control of Rostov-on-Don, a key logistical base for Russia's offensive in Ukraine, "would have significant impacts on Russia's war effort in Ukraine."

Kyiv's defence ministry said it was monitoring the situation. Ukraine was also on high alert after a fresh barrage of Russian missiles Saturday, with casualties and damage reported in Kyiv and the central city of Dnipro.

US President Joe Biden was briefed on the situation in Russia and Washington "will be consulting with allies and partners on these developments," National Security Council spokesman Adam Hodge said.

On Friday, Prigozhin said Moscow's forces were retreating in Ukraine's east and south following the start of Kyiv's counteroffensive early this month. That directly contradicted Putin's account that Ukraine was suffering "catastrophic" losses and that there was a lull in fighting.

"We are washing ourselves in blood," Prigozhin said.

"No one is bringing reserves. What they tell us is the deepest deception," he added, referring to the Russian military and political leadership.

Questioning military operation

After years of operating in the shadows, Prigozhin has now admitted to running the elusive mercenary group Wagner and even interfering in US elections.

His forces, bolstered by tens of thousands of prison recruits, played a central role in Russia's capture of Bakhmut in the longest and bloodiest battle of the conflict.

However, this week he accused Moscow's top brass of deceiving Russians about the offensive in Ukraine.

"Why did the special military operation begin?" he said. "The war was needed for the self-promotion of a bunch of bastards." (AFP)