The head of Russia's private Wagner militia, Yevgeny Prigozhin, said Ukraine was preparing for an "inevitable" counter-offensive and was sending well-prepared units to the devastated eastern city of Bakhmut, for many months the focal point of fighting.

In a combative audio message posted on his Telegram channel, Prigozhin also renewed his criticism of Russia's security establishment, saying there was a "betrayal" going on inside Russia, as he lambasted the defence ministry for not sending much-needed shipments of ammunition to his fighters.

"Today, well-trained enemy units are already being thrown into Bakhmut... A counteroffensive by the Ukrainians is inevitable," Prigozhin said, adding that he expected that campaign to start sometime after May 2 when the weather has improved and the ground has hardened.

Prigozhin - who has for months fought openly with Defence Minister Sergei Shoigu over the conduct of the war - said his troops were suffering five times as many casualties due to a lack of support from Moscow.

He also questioned why Russian forces had not launched offensives against the nearby cities of Sloviansk or Kramatorsk in eastern Ukraine to relieve the pressure on Bakhmut.

But he said his forces would do whatever it takes to halt any Ukrainian attempt to retake the city.

"We will advance at any cost, just to grind down the Ukrainian army and disrupt their offensive," Prigozhin said.

Russia has said capturing Bakhmut will allow it to mount further offensives in eastern Ukraine. Despite downplaying its strategic significance for Moscow, Ukrainian President Volodymyr Zelesnkiy has repeatedly refused to withdraw his forces.

The capture of the city - which Prigozhin said on April 11 was more than 80% controlled by his Wagner forces - would be Russia's first major victory in the conflict since a series of major withdrawals from the northeastern Kharkiv region and southern Kherson region last autumn.