A human rights group says it has identified hundreds of sites where witnesses say North Korea carried out public executions and extrajudicial state killings as part of an arbitrary and aggressive use of the death penalty designed to intimidate its citizens.

The Transitional Justice Working Group (TJWG), based in Seoul, said it had pinpointed at least 323 sites after four years of research and interviews with more than 600 North Korean defectors.

“Public executions are to remind people of particular policy positions that the state has,” said the TJWG research director, Sarah A Son. “But the second and more powerful reason is it instils a culture of fear among ordinary people.“

Purged members of the elite have been among those executed in public, such as leader Kim Jong-un’s uncle, Jang Song-thaek, in 2013.

But the most common charges levelled against the condemned ranged from “stealing copper and livestock” to, less frequently, “anti-state” activities and illegally crossing into China, the group said.

The survey of 610 North Korean defectors living in South Korea included 19 reports of more than 10 people being executed at the same time.

Crowds, often of hundreds of people, and sometimes 1,000 or more, would gather to watch the executions. The youngest person to witness a public execution was seven, the group said.

The group found that 35 reports of public executions came from one particular river bank, with executions taking place at the unidentified location every decade since the 1960s. Six of the executions were by hanging and 29 by firing squad, the group said.

Reuters was unable to independently confirm any of the accounts in the report.

The TJWG is a non-government organisation founded by human rights advocates and researchers from South Korea and four other countries. The group released a report in 2017 based on a smaller number of interviews. It said the latest report was better sourced.

The new report was made possible by funding from the Washington-based National Endowment for Democracy, which is funded by the US Congress.

Son said most of the testimony was based on accounts from defectors who had witnessed executions first-hand, or who had heard about them directly from a witness. “There is no room in the report for rumour or hearsay, or anything that could be seen as vague,” she told the Guardian.

Son said defectors interviewed by the group included officials from the ministry of people’s security and the ministry of state security, as well as party secretaries and members of local community organisations. In some cases, defectors interviewed by TJWG had helped move bodies.

Son added: “Third-party verification is very hard to get. The closest we get is when our research aligns with that of other organisations. There is also a delay in the information getting to us because of the time it takes for defectors to leave and arrive in South Korea, and then we have to find them. But it is getting easier as patterns begin to emerge that go on to indicate more specific trends. It’s important that we continue to expose the diversity of human rights abuses in North Korea.”

The group said 83% of a sample of 84 surveyed people had witnessed a public execution at some time, but it did not give specific data on how common such executions might be, or whether they were getting more or less frequent.

The group warned that the survey sample based on the testimony of defectors was not necessarily representative.

For example, a disproportionate number of the respondents come from northern provinces with the greatest access to the Chinese border for people trying to defect.

Last month, reports emerged of a purge of senior North Korean officials involved in Pyongyang’s nuclear negotiations with Washington, with a claim that one, Kim Hyok-chol, had been executed. The report was later called into question when some figures who were apparently sent into exile or told to lie low reappeared in public.

North Korean state media have made no comment on the executions report, but have denounced previous criticism of the country’s human rights record as part of a US-led campaign to tarnish the regime and foment political instability.

In a report to the United Nations human rights council in May, North Korea said it “consistently maintains the principle of ensuring scientific accuracy, objectivity and impartiality, as well as protecting human rights in dealing with criminal cases”.

Reuters contributed to this report