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Russian authorities said Saturday that security measures had been tightened in several regions, after the chief of mercenary group Wagner said his forces had crossed the border from Ukraine.

Yevgeny Prigozhin vowed to "go to the end" to topple the Russian military leadership, whom he accused of launching strikes on his men, while the country's prosecutor general said he was under investigation for "armed rebellion".

"A decision has been taken to reinforce security measures in the region," said Igor Artamonov, governor of the Lipetsk region south of Moscow. "I ask everyone to remain calm." The region of Lipetsk is around 400 kilometres (250 miles) south of Moscow.

In the southern region of Rostov, officials asked residents to stay home.

"Law enforcement agencies are doing everything necessary to ensure the safety of residents," Rostov governor Vasily Golubev said on social media. "I ask everyone to stay calm," he said, urging locals to stay home.

Earlier Saturday state news agency TASS reported that Moscow authorities had instituted tighter security measures, and local media published amateur footage showing armoured vehicles deploying throughout the capital, including near the defence ministry.

Prigozhin said earlier his units, which spearheaded an assault in eastern Ukraine, had entered the southern region of Rostov.

The Ukrainian army said Friday it was following infighting in Moscow after the head of the Wagner mercenary group accused Russia's military of attacking one of the private fighting group's bases.

"We are watching," the Ukrainian defence ministry tweeted, while Ukraine's military intelligence chief Kyrylo Budanov said rival Russian factions had begun to "eat each other over power and money."

The White House was following the situation in Russia after the head of the Wagner mercenary group called for an "armed mutiny" against Moscow top brass, a spokesman said Friday, adding that President Joe Biden had been briefed.

"We are monitoring the situation and will be consulting with allies and partners on these developments," National Security Council spokesman Adam Hodge said.