MOSCOW - 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.

Wagner chief claims his forces downed Russian military helicopter

The chief of mercenary group Wagner Yevgeny Prigozhin said early Saturday that his forces had shot down a Russian military helicopter.

"A helicopter has just now opened fire at a civilian column. It has been shot down by units of PMC Wagner," Prigozhin said in a new audio message. He did not provide any details and AFP could not verify the claim.

The founder of Russia’s Wagner mercenary force said that his men had crossed border from Ukraine into Russia and were ready to go "all the way" against the Russian military.

Wagner fighters had entered the southern Russian city of Rostov, Prigozhin said in an audio recording posted on Telegram.

He said he and his men would destroy anyone who stood in their way.

Russia earlier on Friday accused Prigozhin of calling for an armed mutiny after he alleged, without providing evidence, that the military leadership had killed a huge number of his fighters in an air strike and vowed to punish them.

President Vladimir Putin has been informed by Russia's prosecutor general of an attempt by the Wagner mercenary group to stage an armed insurrection, the Kremlin announced Saturday.

"The Prosecutor General of the Russian Federation (Igor) Krasnov reported to Putin about the initiation of a criminal case in connection with an attempt to organise an armed rebellion," Kremlin spokesman Dmitry Peskov said.

A Russian state news agency on Saturday reported that Moscow authorities had instituted tighter security measures after the head of the Wagner mercenary group called for the ouster of Russia's military leadership.

"Security measures in Moscow have been tightened. All critical facilities, government and transport infrastructure objects have been put under reinforced protection," the TASS state-run news agency reported, citing a law enforcement source.

This came after Yevgeny Prigozhin, Wagner's founder, accused Moscow of targeting his forces with deadly missile strikes and vowed to retaliate.

The 62-year-old, whose forces have played a central role in key battles in east Ukraine, said he was launching a march to restore "justice," calling for change at the top of the defence ministry.

Russian state television meanwhile broadcast an unscheduled special report, echoing statements from the FSB security service, which accused Prigozhin of fomenting a "civil conflict" and urged Wagner fighters to detain him.

Local media published amateur footage showing armoured vehicles deploying throughout the capital, including near the defence ministry.