By Marc Santora and Norimitsu Onishi

KYIV— Torture chambers in the basements of police precincts and school buildings. Witness testimony of abuse, humiliation and murder. Seven foreign medical students from Sri Lanka held captive found alive and freed. A forest filled with graves.

Across the vast stretch of thousands of square miles of land newly reclaimed by Ukrainian forces in the northeastern Kharkiv region, accusations of Russian atrocities by Ukrainian officials mounted Saturday (17) as Ukraine stepped up its calls for a global response. Many of the claims have not yet been independently verified.

Ukrainian President Volodymyr Zelenskyy said the process of exhuming 440 graves in the recaptured city of Izium, the largest mass grave site discovered to date, was continuing, and he cautioned that it was too early to say exactly how many people were buried there or how had they died. But local authorities said the majority of those removed from the ground had met violent deaths.

“There is already clear evidence of torture, humiliating treatment of people,” Zelenskyy said in his nightly address Friday (16). “The world must react to all this.”

The United Nations said it was dispatching a team to assist in the investigation, while the Biden administration warned the Kremlin against using unconventional weapons to reverse the successful counteroffensive by Ukrainian troops.

In an interview with the CBS News program’“60 Minutes” that aired Sunday, President Joe Biden said the United States’ response to any use of such weapons would be “consequential,” although he did not provide details.

Across a vast front line that has been reshaped after a Ukrainian offensive drove the Russians out of the northeast — but one that still stretches over 1,000 miles — Ukraine continued to try and exploit Russian disarray but claimed no significant forward movement. Russia’s offensive operations appeared to be limited to a small pocket of the Donetsk region around the city of Bakhmut.

Ukrainian soldiers and commanders believe Bakhmut is in an increasingly tenuous position as Russian forces press from the east and southeast in an attempt to cut off the country’s supplies.

Soldiers on the front line around the city have claimed that Russian forces in the area are mainly composed of troops from the Wagner Group, a private military company with ties to the Kremlin. Wagner troops have fought in places such as Syria and Libya — countries with a history of Russian intervention — and Ukrainian soldiers say they are deploying Russian prisoners onto the front lines.

That the Ukrainian forces were being attacked by inmates — and not just regular Russian ranks — suggested a reason there seemed an unending supply of soldiers around Bakhmut attacking them, Ukrainian troops said.

On Tuesday (13), a video posted online and analyzed by The New York Times shows the Wagner Group promising convicts that they will be released from prison in return for a six-month combat tour in Ukraine. It is unclear when the video was filmed.

Even as Ukraine’s security services worked to secure towns and villages seeded with mines left by the Russian forces as they made a chaotic and hasty retreat, teams of Ukrainian investigators were fanning out to take witness testimony of atrocities, collect evidence and start building war-crimes cases.

Ukrainian officials said they had found at least 10 torture chambers, two in the town of Balakliia, including one in a police station.

“During the inspection of the building, we discovered wires leading to hidden video cameras in the rooms where the hostages were kept,” said Serhii Bolvinov, chief of the investigative department of the police of Kharkiv region. “People, including women, had to sleep on the floor, and during interrogations, they were tortured with electric current.”

He said the police had confirmed the death of at least one person because of torture.

As with many of the emerging claims, it was not possible to immediately verify some of the reports. But evidence of many of the crimes was already being made public, and those who survived are now being interviewed as potential witnesses.

They include seven citizens of Sri Lanka, students of Kupyansk Medical College. In March, they had been captured by Russian soldiers and subsequently kept in a basement, according to Zelenskyy, who spoke about their case in his nightly address to the nation.

With Russia suffering military setbacks in the past weeks, and the Kremlin growing increasingly isolated diplomatically, the Biden administration warned President Vladimir Putin against using unconventional weapons, marking the second time that Washington has issued such a warning. In March, Biden said that “we would respond” if Putin — then frustrated by setbacks in Kyiv, the Ukrainian capital, and by the general lack of progress in the war — were to use such weapons.

In the ‘60 Minutes’ interview, Biden said Russia’s use of unconventional or nuclear weapons to try to turn the tide of the war in Moscow’s favour would “change the face of war unlike anything since World War II.”

This time, the warning came as Russian forces have been left reeling from their retreats on the battlefield and as Putin faces intensifying questions back home over how he has conducted the war.

The leaders of Putin’s two most consequential strategic partners, India and China, both raised concerns about the war this past week, puncturing the Kremlin’s message that Russia was far from isolated as a result of the war.

Some Western officials have expressed concern that the more cornered Putin feels, the greater the chance that he might turn to an unconventional weapon such as a tactical or low-yield nuclear weapon, which can be fired at relatively short distances, as opposed to “strategic” nuclear weapons that can be launched over much longer distances.

In April, CIA Director William Burns warned about how Putin could turn to such weapons in “desperation.”

Burns said it was a possibility that the United States remained “very concerned” about, although he said that, at that stage of the war, Washington had seen no “practical evidence” of the kinds of military deployments or movement of weapons to suggest that such a move was imminent.

Despite the setbacks and the loss of tens of thousands of Russian soldiers in Ukraine, Putin has showed no signs of changing course. On Friday, he threatened to escalate his forces’ attacks.

In a news conference in Uzbekistan at the conclusion of a regional meeting, Putin claimed that Ukraine was trying to carry out “terrorist acts” inside Russia and “to damage our civilian infrastructure.”

Ukraine has occasionally hit fuel and military targets in Russia’s border region but has denied targeting civilian infrastructure, and Putin offered no evidence to back up his assertion.

Zelenskyy has sought to seize on his country’s military advances to bolster the resolve of Western allies, using them as proof that Ukraine is capable not only of mounting an effective defence but also of driving Russian forces from the country and winning the war.

In the northern Kharkiv region, Ukrainian forces continued to consolidate their gains after their lightning counteroffensive against Russian troops, according to the Pentagon’s chief spokesperson.

“In the north, what we assess is that the Ukrainians are consolidating their gains after taking back significant territory and that the Russians are attempting to shore up their defensive lines after having been pushed back,” the spokesperson, Brig. Gen. Patrick Ryder, told reporters Friday at the Pentagon.

“In the south, the Ukrainians continue to make what we would assess as deliberate, calculated forward movement as the Russians continue to try to hold that line,” he said.

Ukrainian and Russian forces traded long-distance blows in southern Ukraine on Friday, bombing each other’s positions and inflicting major damage.

The two top floors of the city administration building in Kherson, a southern Ukrainian city the Russians occupy, were turned into rubble by a Ukrainian missile strike. At least three people died, according to the Russian news agency TASS. The Ukrainians took credit for the attack, saying it was an attempt to decapitate the leadership of the occupied area.

“All the collaborators of the occupied Kherson region were gathered there,” said Serhiy Khlan, a regional legislator. “When they all gathered, a ‘greetings’ from the Armed Forces of Ukraine arrived.”

Khlan warned civilians to stay away from government offices and military targets in Kherson, in case there were more strikes.

-New York Times