Allegations of torture and disappearances are widespread in Russian-occupied Ukrainian city of Kherson

It has been 100 days since Russia invaded Ukraine, and Ukrainian President Volodymyyr Zelenskyy says Russian forces now control 20 percent of the country, describing those areas as a "complete disaster."

After Russia took over the southern city of Kherson, Olexander Guz, a former Ukrainian military draftee, said he was tortured by Russian forces — hung by his wrists and put in stress positions.

He has now escaped to Odesa, and while his wounds have begun to heal, his memories are still fresh.

"They put a bag over my head and then began beating me," Guz said. "I was so severely beaten, I lost consciousness."

Kremlin-backed forces seized control of Kherson, Guz's hometown, in the early days of Russia's invasion. Since then, Moscow's grip on the city of nearly 300,000 has only tightened, and allegations of torture and disappearances are widespread.

Those who have tried to escape recently have been turned back by Russian forces.

"You feel constantly under threat that Russians will come and take you away," Bogdan Andrienko, a 20-year-old volunteer providing aid to people in Kherson, told CBS News over an encrypted video call. He said he was afraid of what would happen to him.

"A bunch of volunteers in the city were already taken by the Russians, and we don't know what happened to them," Andrienko said.

Every so often, video emerges from Kherson of Ukrainian civilians resisting Russia's occupation, and then being met with violence. Those who make it out take shelter in cities like Odesa, to the south, where they receive aid and other support.

"I knew I had to leave the town I was born in, but I did it for my son," Olha Chumachenko, who fled Kherson for Odesa with her four-year-old over a month ago, told CBS News. "He's my only reason for living," she said.

Ukrainian forces have started training on the long-range rockets promised this week by President Biden. They have vowed to take back all of Ukraine's territory that is under Russian control, like Kherson.