Some officials worry that the more cornered Vladimir Putin feels, particularly with recent setbacks from Ukraine’s counteroffensive, the greater the chance that he might turn to an unconventional weapon.

KYIV, Ukraine — 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 on Saturday as Ukraine stepped up its calls for a global response. Many of the claims have not yet been independently verified.

President Volodymyr Zelensky 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 the 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,” Mr. Zelensky said in his nightly address on Friday. “The world must react to all this.”