Russian mercenary boss Yevgeny Prigozhin, who has been arguing with top military brass for months, on Friday escalated the feud by accusing pro-Moscow forces of trying to blow up his men.

Prigozhin’s Wagner Group troops have largely pulled back from the eastern Ukrainian town of Bakhmut, most of which they captured last month after taking heavy casualties, and handed over their positions to regular Russian forces, Reuters reported.

Prigozhin, writing on Telegram, said his men had discovered a dozen locations in rear areas where defense ministry officials had planted various explosive devices, including hundreds of anti-tank mines. When asked why the charges had been set, the officials indicated it was an order from their superiors.

“It was not necessary to plant these charges in order to deter the enemy, as it (the area in question) is in the rear area. Therefore, we can assume that these charges were intended to meet the advancing units of Wagner,” he said.

None of the charges went off and no one was hurt, he said, adding: “We assume this was an attempt at a public flogging.”

Russia’s defense ministry was not immediately available for comment.

Prigozhin, who regularly complained his men were not given enough ammunition for the assault on Bakhmut, said on Wednesday that he had asked prosecutors to investigate whether senior Russian defense officials had committed any “crime” before or during the war in Ukraine.

U.S. President Joe Biden predicted on Thursday Sweden will join NATO “soon”, speaking at the U.S. Air Force Academy days after he hinted at a possible deal to overcome Turkey’s opposition to admitting the Nordic country to the alliance, Reuters reported.

Biden, in a flag-waving commencement address in Colorado Springs, Colorado, warned graduates they will enter service in an increasingly unstable world, citing challenges from Russia and China.

Biden, 80, who stood for much of the hours-long ceremony to shake hands with graduates, appeared to trip over something and fell to his knees towards the end. He was assisted to his feet and walked to his seat unaided.

On Monday, Biden spoke to Turkish President Recep Tayyip Erdogan to congratulate him on his re-election. Biden told reporters Erdogan repeated Ankara’s desire to buy F-16 fighter jets from the United States, while Biden urged Ankara to drop its objection to Sweden’s joining NATO, read the report.

In his commencement speech, Biden said NATO is stronger in spite of Russian President Vladimir Putin’s attempt to crack the alliance with his invasion of Ukraine. It is bolstered further, he said, by the recent admission of Finland, “and soon, Sweden.”

“It will happen, I promise you,” he said, but provided no details.

On Monday, Biden said he would speak to Erdogan again soon. NATO’s annual summit is in Vilnius, Lithuania, in July, Reuters reported.

On Tuesday, U.S. Secretary of State Antony Blinken urged Turkey to immediately finalize Sweden’s accession to NATO, saying the country had already taken significant steps to address Ankara’s objections to its membership.

The White House has denied that Biden is pursuing a deal with Turkey to lift its opposition in exchange for F16s.

The 80-year-old president, who is seeking re-election in 2024, stood, throughout the presentation of more than 900 diplomas despite the relatively thin mountain air, shook hands and gave sharp salutes to the blue-uniformed graduates. His fall came at the end of the individual salutes. The Air Force Academy stands at 7,258 feet (2,212.3 meters) above sea level, Reuters reported.

Biden made clear the United States would not back down from the challenge posed by China amid deep strains in the relationship.

“The United States does not seek conflict or confrontation with China. China and the United States should be able to work together where we can to solve some global challenges, like climate,” he said.

“But we are prepared for vigorous competition,” he said, adding the United States would stand up for its interests and that of its partners.

The president emphasized U.S. support for Ukraine in the war with Russia, Reuters reported.

“The American people’s support for Ukraine will not waver,” Biden said.

Drones attacked two oil refineries just 40-50 miles (65-80 km) east of Russia’s biggest oil export terminals on Wednesday, sparking a fire at one and causing no damage to the other, according to Russian officials.

Drone attacks deep inside Russia have intensified in recent weeks with strikes on Moscow, oil pipelines and even the Kremlin ahead of a Ukrainian counter-offensive.

At around 0100 GMT a drone struck the Afipsky oil refinery in Russia’s Krasnodar region, causing a fire which was later extinguished, Governor Veniamin Kondratyev said.

The Afipsky refinery lies 50 miles east of the Black Sea port of Novorossiisk, one of Russia’s most important oil export gateways. The plant can process around 6 million tonnes (44 million barrels) of oil each year, Reuters reported.

Novorossiisk, together with the Caspian Pipeline Consortium (CPC) terminal, bring about 1.5% of global oil to market.

Last year, CPC exported via the South Ozereyevka terminal 58.7 million tonnes of oil, mainly from Kazakhstan, while the terminal of Sheskharis at Novorossiisk handled about 30 million tonnes of oil.

Another drone crashed into the Ilsky refinery, which lies around 40 miles east of Novorossiisk, Russian state-owned news agency RIA reported, citing local officials.

According to the refinery’s web site, its five processing units have the combined capacity of 3 million tonnes per year.

There was no immediate information on who launched the drones but Russia has accused Ukraine of increased attacks on targets inside the country, including on Moscow on Tuesday, read the report.

Ukraine almost never publicly claims responsibility for attacks inside Russia or on Russian-controlled territory in Ukraine.

Refineries across Russia have been frequently attacked by drones following the start of what the Kremlin casts as the “special military operation” in Ukraine in February 2022.