North Korea has increased the size of a labor camp where prisoners have been beaten to death with hammers and forced to dig their own graves, according to a report by a rights group published Thursday.
Amnesty International commissioned satellite analysis of the country's largest prison camp -- which is known as kwanliso 16.
It shows new buildings have been constructed inside the compound -- which is three times the size of Washington, D.C. -- since North Korean leader Kim Jong Un replaced his late father.
Amnesty International also interviewed guards and inmates who have first-hand experience of life in the camps. They said women are often raped and then executed in secret by officials, and those who try to escape are beaten before being publicly shot or hanged.
Inmates -- including children -- are made to work long hours in dangerous logging and mining jobs in which many of them die.
"The prisoners are only humans insofar as they can speak," said a former prison official who was not named in the report. "In reality though, they are worse off than animals. The purpose of prison camps is to oppress, degrade, and violate the inmates for as long as they are alive."
According to report, more than 100,000 people are imprisoned in labor camps for alleged crimes against the state, which can include "gossiping" about Kim or his predecessors.
The North Korean government denies the existence of the camps, including those repeatedly observed by satellites.
Amnesty International commissioned the satellite analysis in October.
As well as the expansion of the 348 square mile kwanliso 16, near Hwaseong in North Hamgyong province, the report said houses had been demolished at the smaller kwanliso 15, known as "Yodok."
Amnesty International said the decrease in housing could indicate a slight reduction in the camp's population. But the group was not able to verify the prisoner population or the fate of its detainees.
A guard referred to in the report as only "Mr. Lee" worked at kwanliso 16 in the mid-1980s until the mid-1990s. He told Amnesty International in an interview in November that he had witnessed inmates being forced to dig their own graves before being killed by hammer blows to the neck.
Women were raped by officials and then never seen again, he alleged.
Kim Young-soon said he was imprisoned in Yodok between 1980 and 1989.
He was accused of gossiping about former leader Kim Jong Il, and like all North Korean prisoners was given no trial. Family members were also sent there, deemed guilty by association.
"Upon arriving in Yodok, it felt like the sky was collapsing," he told Amnesty International. "I couldn’t understand how this could happen to me. How can I be taken to such a deplorable place? It was heartbreaking."
He described the scene of one of the executions: "The prisoner is first beaten half to death. He is tied to a pole up on a platform, with his hands tied behind his back. His feet are also tied, another rope is tied around his waist, and he is blindfolded.
"Then one guard shouts to the firing squad, 'In the name of the people, shoot the enemy of the revolution!' They shoot three shots to the head, three shots to the chest and three to the legs. By then, the head drops and the body is dragged away."
Inmates are forced to work by guards who withhold their meager food rations, the report said.
One couple detained in kwanliso 15 between 1999 and 2001 described the brutal labor conditions at their camp.
"During the course of our three-year detention, often we did not meet our targets because we were always hungry and weak," Lee and Kim -- whose full names were withheld -- told Amnesty International. "We were punished with beatings and also reductions in our food quota."
Amnesty International's East Asia researcher Rajiv Narayan said North Korea's human rights violations were "systematic, widespread and very grave." The group has urged the North Korean government to recognize and decommission the labor camps.
A U.N. report in September documented the "unspeakable atrocities" faced by people kept in North Korean labor camps. It said it interviewed people forced to eat lizards and rodents to survive, and a woman who saw another inmate forced to drown her baby in a bucket.