As Russia vowed to respond “extremely hard” to a rare two-day border incursion by pro-Ukraine fighters, the leader of Russia’s largest mercenary force warned it faced further setbacks unless its ruling elite took drastic, and likely unpopular, measures to win the war.

“The most likely scenario for us in a special operation would not be good,” Yevgeny V. Prigozhin, the founder of the Wagner mercenary group, said in a profanity-laced interview with a pro-Kremlin political observer published late Tuesday on the messaging platform Telegram. “We are in such a state that we could lose Russia,” he continued with profanity in his speech. “We must prepare for a very hard war that will result in hundreds of thousands of casualties.”

An oligarch closely allied with Russian President Vladimir V. Putin, Mr. Prigozhin has increased pressure on Russia’s military leadership with bombastic statements on public Internet platforms, extending his criticism to the country’s money elite.

He has gained additional power from his infamous mercenary forces role in the recent capture of Bakhmut, Russia’s first battlefield victory in months. However, Russian state media have kept his name out of their coverage of these events, showing how Russia’s propaganda machine has been hiding elite battles and frontline problems from the Russian people.

In the interview, Prigozhin called for all-out war – something Putin has carefully avoided, trying to reassure his people that their lives will not be disrupted by the “special military operation” in Ukraine. That position has become harder to maintain as the war drags on and Russian losses mount.

The Kremlin, Prigozhin said, must announce a new wave of mobilization to call up more fighters and declare martial law, forcing “everyone” into the country’s munitions production.

“We must stop building new roads and infrastructure facilities and just work for the war, to live a few years in North Korea’s image,” he said. “If we win, we can build anything. We stabilize the front and then move on to some kind of active action.”

The alternative, he said, is more violence, but inside Russia, carried out by ordinary people fed up with elites, who Mr Prigozhin characterized as ignoring the reality of the war but not doing enough to win it.

“The children of the elite smear themselves with creams, show it on the Internet, the children of ordinary people come in zinc, torn to pieces,” he said, referring to the coffins of dead soldiers, adding that those killed in action had “tens of thousands” of relatives. “Society always demands justice, and if there is no justice revolutionary feelings arise.”

Mr Prigozhin said his Wagner force alone had lost 20,000 men during the war in Ukraine, half of whom had been recruited from prisons. These convicts represent 20 percent of the total number of imprisoned convicts who joined the fighting force.

A State Department spokesman, Matthew Miller, said the United States believed Prigozhin’s number was a significant underestimate of his losses. Even so, it is significantly higher than the losses of the Russian Armed Forces that the Kremlin has acknowledged. While US estimates range significantly higher, the Russian government has only acknowledged the deaths of 6,000 soldiers – statistics that were last shared publicly in September.

Mr Prigozhin’s comments in the interview came after an incursion into Russia’s Belgorod region by Ukrainian militants. The fighters, ethnic Russians seeking Ukraine’s victory, apparently used US-made armored vehiclesand instigated the fiercest fighting on Russian soil since the war started 15 months ago.

Mr Prigozhin said Ukraine has “one of the strongest armies in the world”, adding that the border violence reflected poor leadership at the highest level of the Russian military. He has often singled out Defense Minister Sergei K Shoigu as the object of his ire, and in the interview Prigozhin defined his personal credo as: “I love my motherland, I serve Putin, Shoigu should be convicted and we will fight on.”

In brief comments during a meeting with colleagues on Wednesday, Mr Shoigu offered no reaction to Mr Prigozhin’s comments, insisting Russia would “react quickly and extremely hard” to any further incursions by “Ukrainian militants”.

Many analysts and other observers marvel at Mr. Prigozhin’s regular rants against Russia’s elite in a tightly controlled society, and especially his targeted criticism of Mr. Shoigu.

“He’s playing a very dangerous game,” a wealthy Moscow-based businessman said of Mr. Prigozhin in an interview with The New York Times in late March, requesting anonymity to discuss a prominent Kremlin-connected individual. “If he doesn’t stop, he will end up like Aleksei Navalny.” Mr Navalny, Russia’s most prominent opposition politician, is now in poor health in a penal colony.

But Wagner’s latest victory in Bakhmut after a grueling months-long battle has given Mr Prigozhin political carte blanche, said Dmitri Oreshkin, a Russian political scientist and Kremlin critic.

“You get everything, permission to break the law, to take people from prisons without asking anyone’s permission, to kill those people if you don’t like them for discipline,” said Mr. Oreshkin on the terms of the agreement between Mr. Putin and Mr. Prigozhin. “If he hadn’t taken this victory, he would have been torn apart” by the elite he has disparaged.

“For him it was a matter of life and death.”

Milana Mazaeva contributed reporting.