Oleg Kotenko, the Commissioner for Issues of Missing Persons under Special Circumstances, looks at the unidentified graves of civilians and Ukrainian soldiers in the recently retaken area of Izyum, Ukraine, September 15, 2022. EVGENIY MALOLETKA/AP

The head of the NATO alliance cautioned Friday that while the stunning counteroffensive by Ukraine's forces, which has seen them claw back a huge swath of ground previously occupied by Russian troops, is "extremely encouraging," it is "not the beginning of the end of the war."

NATO Secretary-General Jens Stoltenberg said Ukraine's partners "need to be prepared for the long haul," as Ukraine's forces push their gains and Russia shows no sign of halting its assault further to the east and the south.

As CBS News foreign correspondent Debora Patta reports, the full horror of that assault becomes clearer by the day, as residents of towns and villages in the recently-liberated Kharkiv region emerge from months in hiding.

As Russian forces seized Izyum, they laid waste to the town. Vladimir Putin's military shelled Izyum relentlessly for weeks before they fully wrested control of the town and turned it into a logistics hub.

The shelling destroyed hospitals, homes and schools — clearly not military targets. Many people were killed, often buried hastily in shallow mass-burial sites.

Patta visited a makeshift cemetery in Izyum on Friday where the bodies of hundreds of civilians were laid to rest in shallow graves during Russia's occupation. Many died in their homes as artillery rained down. Others were executed.

The awful task of exhuming and identifying the bodies has begun. It will take days, and each victim exhumed will be a personal trauma for family and friends, and another national tragedy for war crimes prosecutors to investigate.

Once they controlled Izyum, the Russian forces also attempted to erase every trace of Ukrainian identity in the town.

"They burned our books, destroyed our schools, removed our TV channels and put on Moscow's propaganda," resident Yulia Koziubenko told CBS News.

Anyone with connections to Ukraine's military forces was subjected to torture by the Russians, residents told Patta.

"Those who served in the security services were found," Koziubenko said. "Taken away and beaten."

The bodies found in the shallow graves show that in some cases, they met an even worse fate.

But for some, even harder to bear than the loss of their culture, their town and their neighbors, has been the knowledge that some of their own betrayed them.

Koziubenko said her husband was among them. She has denounced him as a traitor.

"My husband is working with the Russians in Kherson," she said, referring to a region further to the south still largely occupied by Russia. "He no longer exists for me."

There were those who say they had no choice but to take what Russia gave them, including Svetlana Fisher. The Izyum resident told Patta the town's mayor abandoned them, and she had nowhere else to turn for food.

"I couldn't leave. Nobody helped me," she protested angrily. "Now I am a traitor because I had to survive on Russian rations?"

Others stood firm, refusing handouts from the occupiers. Liza Jankina and 50 of her neighbors survived for more than 100 days with hardly any food in a freezing basement. They would often go weeks without a proper meal during Russia's nearly-six-month-long occupation of Izyum.

Now she's bitter about anyone in the town who was seen to have supported the Russian troops.

But as the full extent of Russian atrocities is revealed in Izyum, that bitterness is quickly turning into hatred for the people who brought death to the town.

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