North Korea is demanding the United States "rolls back its hostile policy" and warns their nuclear deal could be "dead" ahead of the anniversary of the historic Trump-Kim summit.
"Now is the time for the U.S. to withdraw its hostile policy" concerning North Korea, the statement reads.
Mr Kim had been a known critic of his family's reign in North Korea.
The Transitional Justice Working Group is a nongovernment organization founded by human rights advocates and researchers from South Korea and four other countries.
". now is the time for the U.S.to withdraw its hostile policy concerning the DPRK", it said.
In 2013, popular North Korean singer Hyon Song-wol was alleged to have been publicly executed, with a South Korean newspaper saying she was shot "in a hail of machine gun fire while her orchestra looked on". It has previously been suggested that Kim had been working with American intelligence agents, though the Journal's report on the supposed relationship is the most detailed to date.
"Since 2005, public hangings are reported to have ceased or at least decreased markedly in frequency, with some attributing this shift to worldwide pressure to end the practice", the report said.More news: Danny Rose uncertain of Spurs future
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The first-ever meeting between a North Korean leader and a sitting USA president took place on June 12 a year ago in Singapore, where Kim Jong-un and Donald Trump signed a vaguely-worded deal to achieve "complete denuclearisation".
Following a provocative run in nuclear and missile tests, Kim initiated diplomacy with Washington and Seoul in 2018 in attempting to leverage his arsenal for economic and security benefits.
The vast majority of executions happen by firing squad, defectors said.
The report claimed sham trials were "almost always" carried out on the spot ahead of a public execution, with victims often appearing "half-dead" upon arrival. Intent to kill is crucial to a murder charge under Malaysian law.
Details of the exact relationship between Kim and the Central Intelligence Agency remain unclear. The bodies and burial locations of those killed were rarely given to their relatives. Some defectors reported incidents in the mid-2010s where guards used metal detectors to find and confiscate mobile phones from witnesses to prevent them from recording the events, which showed the government's concern about the information on public executions getting outside the country, the report said.
Transitional Justice Working Group technology director Dan Bielefeld said the group meant to use the gathered data to pursue accountability for North Korea's human rights abuses, and to support redress for victims.
Ethan Shin, one of the authors of the report, said the study indicated a decline in death sentences in the reclusive regime.