The head of Wagner mercenary group Yevgeny Prigozhin announced Saturday that he was inside the army headquarters in Rostov-on-Don in southern Russia, and that his fighters controlled the city's military sites.

Prigozhin earlier said his forces had crossed into Russia from the Ukrainian front, vowing to topple Russia's military leadership, saying he and thousands of his fighters were "ready to die."

Russia's military headquarters in Rostov-on-Don are a key logistical base for its offensive in Ukraine.

"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.

He said that planes taking part in the Ukraine offensive "are leaving as normal" from the airfield.

"We took (the aerodrome) under control so that the attack aviation did not strike us, but strike Ukrainians," Prigozhin said.

He called on Russians not to believe what they are being told on state television.

"When they tell you that PMC Wagner interfered with work and that's why something on the front collapsed.. Things on the front collapsed not for this reason," he said, addressing Russians.

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

Prigozhin claimed Russia's Chief of General Staff Valery Gerasimov "ran away from here when he found out that we are approaching the building."

Authorities in Rostov have urged residents to stay at home.

Russia opened a criminal case against Prigozhin, accusing him of trying to start an "armed rebellion."