SEOUL: Tens of thousands of people in North Korea, South Korea, Japan, and China could be exposed to radioactive materials spread through groundwater from an underground nuclear test site, according to a report released on Tuesday by a Seoul-based human rights group.

According to the US and South Korean governments, North Korea secretly conducted six nuclear weapon tests at the Punggye-ri site in the mountainous North Hamgyong Province between 2006 and 2017.

According to the Transitional Justice Working Group study, radioactive materials could have spread across eight cities and counties near the site, where more than 1 million North Koreans live and groundwater is used in daily life, including drinking.

It also stated that agricultural and fisheries products smuggled from the North could endanger neighbouring South Korea, China, and Japan.

The group, which was formed in 2014, collaborated with nuclear and medical experts and defectors for the study, which was supported by the National Endowment for Democracy, a non-profit corporation funded by the United States Congress.

"This report is significant in showing that North Korea's nuclear tests could endanger not only the North Korean people's right to life and health, but also those in South Korea and other neighbouring countries," said Hubert Young-hwan Lee, the group's chief and a co-author.

Reuters' phone calls to North Korea's diplomatic mission to the United Nations in New York were not returned.

South Korea's food safety agency discovered nine times the standard level of radioactive caesium isotopes in imported hedgehog mushrooms sold as Chinese produce but originating in North Korea in 2015.

Following the North's previous nuclear tests, China and Japan increased radiation monitoring and expressed concerns about potential exposure, but they did not openly provide information on contaminated food.

Many outside experts have expressed concern about potential health risks from contaminated water, but North Korea has dismissed such concerns, claiming that there have been no leaks of harmful materials as a result of previous nuclear tests without providing evidence.

North Korea confiscated the radiation detectors of foreign journalists who were invited to witness the destruction of some tunnels at the nuclear test site in 2018.

Since 2018, Seoul's Unification Ministry, which handles inter-Korean affairs, has stopped testing defectors for radiation exposure as cross-border ties have thawed.

However, at least nine of the 40 defections from the areas close to Punggye-ri that underwent radiation testing in 2017 and 2018 manifested abnormalities. However, the ministry claimed that it was unable to establish a direct connection with the nuclear facility.

Since 2006, more than 880 North Koreans have reportedly escaped from those areas, according to the report.

The rights organization demanded that testing resume and that a global investigation is conducted into the radiation risks to the areas around Punggye-ri.

An inquiry for comment was not immediately answered by the Unification Ministry.

Pyongyang may be getting ready for its seventh nuclear test, according to Seoul and Washington.