In a joint letter delivered to the panel on Tuesday, groups of North Korean defectors, including one called Free the NK Gulag, said they hoped that the inquiry would lead to the indictment of “Kim Jong-un and his clique” in the International Criminal Court.

“We ourselves or our family members were dragged into prisons without trial, we suffered unspeakable torture and humiliation when we were caught while trying to flee North Korea, and we had to witness neighbors and relatives dying while waiting for food rations,” their statement said. “We were forced to witness almost monthly public executions. Because of our grandfathers’ background, many of us were deprived of an opportunity to join the party or get jobs.”

Jee Heon-a, 34, the other defector who testified on Tuesday, said that during a famine in the late 1990s, North Korean women were sold to traffickers in China. Those later caught by the Chinese police were repatriated to the North. Many suffered forced miscarriages through beatings and other forms of torture from North Korean guards at detention facilities, and one woman who gave birth was forced to drown her baby, she said.

Mr. Kirby said his panel would “seek to determine whether crimes against humanity have occurred and who bears responsibility.” But he cautioned, “It is not possible at this moment to envisage the level of detail that the commission will be able to achieve in establishing lines of responsibility, if any.”

There was poignancy in the fact that Tuesday’s hearing was being held in South Korea. Here, North Korean human rights concerns have never elicited the kind of fervent activism seen in the days when South Koreans fought for their own rights under military dictatorships. Until recently, the authorities here feared that without an effective means of pressuring North Korea, openly challenging the North on human rights would only make it more repressive and less willing to engage in international efforts to denuclearize the country.

Mr. Shin, whose stories of his life in the North Korean labor camp — including watching his mother and brother be executed — are chronicled in the book “Escape from Camp 14” by Blaine Harden, said Tuesday that he had met people who questioned the veracity of his testimonies. He said he had no evidence except his accounts and the scars that he suffered from torture and from an electrified fence that he crossed to escape the camp.

“I want to speak for those children who were born and live in prison camps as I did, and for people who don’t even know what human rights are,” he said. “I hope that they can say they are hungry when they are hungry and say it’s painful if someone hurts them.”