The Transitional Justice Working Group (TJWG) found 323 reports of state-sanctioned killing sites - all of which the NGO said it has geographical coordinates for, Efe news reported.
The findings were released in a report titled "Mapping the Fate of the Dead: Killings and Burials in North Korea", which also documented 318 reports of public execution sites.
The public executions, said to have taken place in every decade since the 1960s, occurred in places such as river beds and river banks, open spaces and fields, marketplaces, mountains, sports grounds and local schools for crimes ranging from murder, human trafficking, stealing cows and watching the South Korean media, the report said.
"Almost all of the state-sanctioned killings reported were public executions by firing squad. Brief 'trials' almost always occur on the spot immediately before a public execution, where charges are stated and a sentence given without legal counsel for the accused," it added.
Some executions were witnessed by over 1,000 people, including children, and the families of those sentenced were sometimes forced to watch, the NGO said.
Interviewees described instances where those responsible for carrying out executions covered their faces, presumably to hide their identity, or appeared to be drunk.
One interviewee described seeing about 80 labour camp inmates - who had been caught trying to cross to China or who had been repatriated from the country - forced to watch the killing of three women accused of brokering escapes from North Korea.
Some research participants also reported the presence of plain-clothes security officers in the crowd, and their mobile phones being taken from them upon entry, presumably to prevent the executions being recorded and distributed.
Escapees interviewed told the TJWG that investigating killings and body disposals may "help to prosecute the North Korean regime" in the future and that the dead "should be returned to their families as the dead are human beings".
"The inability to access information on the whereabouts of a family member killed by the state and the impossibility of giving them a proper burial violates both cultural norms and the 'right to know,'" said the report's lead author Dr Sarah Son.
( With inputs from IANS )