Russia's most powerful mercenary said on Sunday (June 11) that his Wagner fighters would not sign any contract with defence minister Sergei Shoigu, publicly refusing an attempt by the defence ministry to bring his fighting force under its sway.

Yevgeny Prigozhin, the founder of the Wagner group, has repeatedly attacked President Vladimir Putin's top military brass for what he casts as treachery for failing to fight the war in Ukraine properly.

Neither Shoigu nor Chief of the General Staff Valery Gerasimov have commented in public on the insults from Prigozhin, whose forces in May took the Ukrainian city of Bakhmut after a battle in which tens of thousands perished.

The defence ministry on Saturday said Shoigu had ordered all "volunteer detachments" to sign contracts with his ministry by the end of the month, a step it said would increase the effectiveness of the Russian army.

Though the ministry did not mention Wagner in its public statement, the Russian media reported that it was an attempt by Shoigu to bring the mercenaries to heel.

"Wagner will not sign any contracts with Shoigu," Prigozhin said in response to a request for comment on the matter. The order, he said, did not apply to Wagner.