The head of the Russian mercenary group Wagner, who has been entangled in a power struggle with the defence ministry, announced Friday the opening of recruitment centres in dozens of cities.

Wagner has been spearheading offensives against cities in eastern Ukraine including Bakhmut, in what has become the longest and bloodiest fight of Russia's year-long assault. Both sides have suffered heavy losses around Bakhmut.

"Recruitment centres for PMC Wagner have opened in 42 Russian cities," Wagner chief Yevgeny Prigozhin said in a statement, referring to his private military company.

Fighters will be recruited in sports centres and martial arts clubs, according to a list released by Prigozhin.

"Despite the colossal resistance of the Ukrainian armed forces, we will move forward," the 61-year-old pro-Kremlin businessman said.

He also alluded to the apparent pressure of the Russian defence ministry.

"Despite the spanners that they are throwing in the works at every move, we will overcome this together," he said.

Prigozhin had for months recruited prison inmates, promising amnesty upon their return to Russia if they survived the fighting in Ukraine.

But in early February he said the recruitment from prisons had ended, apparently under pressure from the defence ministry.

Tensions between Prigozhin and the defence ministry have raged for months.

Prigozhin has several times claimed battlefield victories ahead of Russia's army, criticised Russia's top brass and accused the military of not sharing ammunition with his ragtag forces.

Sanctioned by Washington and Brussels, Prigozhin had for years operated in the shadows but has been thrown into the limelight since the start of the offensive in Ukraine.