The leader of the Wagner mercenary group forecasts disaster if Russia does not move into total war footing.

As Russia vowed to respond “extremely harshly” to a rare, two-day border incursion by pro-Ukrainian fighters, the leader of Russia’s largest mercenary force warned that 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 a good one,” 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 Telegram messaging platform. “We are in such a condition that we could lose Russia,” he continued, his speech laced with profanity. “We have to prepare for a very hard war that will result in hundreds of thousands of casualties.”

An oligarch closely allied with President Vladimir V. Putin of Russia, Mr. Prigozhin has been ramping up pressure on Russia’s military leadership with bombastic diatribes on public internet platforms, and extending his criticism to the country’s moneyed elites.

He has been further empowered by his notorious mercenary force’s role in the recent conquest of Bakhmut, Russia’s first battlefield victory in months. However, Russian state media has kept his name out of its coverage of those events, showing how Russia’s propaganda machine has been hiding elite infighting and problems on the front line from the Russian people.

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

The Kremlin, Mr. Prigozhin said, must declare a new wave of mobilization to call up more fighters and declare martial law and force “everyone possible” into the country’s ammunition production efforts.

“We must stop building new roads and infrastructure facilities and work only for the war, to live for a few years in the image of North Korea,” 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, perpetrated by ordinary people fed up with elites, whom Mr. Prigozhin characterized as ignoring the reality of war but not doing enough to win it.

“The children of the elite smear themselves with creams, showing it on the internet, ordinary people’s children come in zinc, torn to pieces,” he said, referencing the coffins of dead soldiers, and adding that those killed in action had “tens of thousands” of relatives. “Society always demands justice, and if there is no justice, then revolutionary sentiments 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. Those convict fighters represent 20 percent of the total number of imprisoned convicts who joined the fighting force.

A State Department spokesman, Mark Miller, said that the United States considered Mr. Prigozhin’s number a significant undercount of his losses. Even so, it is significantly higher than the Russian Armed Forces’ losses that the Kremlin has acknowledged. While American estimates range significantly higher, the Russian government has admitted only the deaths of 6,000 soldiers — statistics last publicly shared in September.

Mr. Prigozhin’s comments in the interview came after an incursion into Russia’s Belgorod region by Ukraine-aligned militants. The fighters, ethnic Russians who seek Ukraine’s victory, apparently used U.S.-made armored vehicles, and instigated the fiercest fighting on Russian soil since the war started 15 months ago.

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

In brief remarks during a meeting with colleagues on Wednesday, Mr. Shoigu offered no reaction to Mr. Prigozhin’s comments and maintained that Russia would “respond promptly and extremely harshly” to any further incursions by “Ukrainian militants.”

Many analysts and other observers marvel at Mr. Prigozhin’s regular diatribes against Russia’s elite in a society that is strictly controlled, and especially his targeted criticisms of Mr. Shoigu.

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

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

“You are given 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,” Mr. Oreshkin said about the terms of the deal between Mr. Putin and Mr. Prigozhin. “If he had not brought this victory, he would have been torn apart” by the elites he has been disparaging.

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

Milana Mazaeva contributed reporting.