The Wagner mercenary group which led Moscow's fight against Ukraine is now taking aim at Russia.

According to Reuters and AFP news agency, the head of the private military contractor, Yevgeny Prigozhin, said Saturday that his men had crossed the border into Russia from Ukraine and were prepared to go "all the way" against Moscow's military.

In an audio recording posted on Telegram, the Wagner boss said the fighters had entered the southern Russian city of Rostov and would destroy anyone who stood in their way. 

Without giving specifics such as a location, he also claimed that his men had shot down a Russian military helicopter.

The mercenary group was fighting for Russia on the front lines of eastern Ukraine. Despite the feat of capturing Bakhmut, the group has been at odds with Russian authorities.

Prigozhin earlier claimed that the Russian military had killed a huge number of his troops and openly called for an armed rebellion.

The Kremlin, which ordered his arrest, accused him of armed mutiny in a briefing to President Vladimir Putin.

Moscow is also stepping up security around key facilities against a possible attack on the defense ministry by the mercenaries.

Meanwhile Ukraine and the United States are keenly monitoring the latest developments including a possible internal conflict brewing among Russian forces.

U.S. National Security Council spokesperson Adam Hodge said the U.S. was keeping close tabs and will consult with allies and partners on the evolving situation, adding that President Joe Biden was also briefed on the matter.