At least four defectors from North Korea have shown signs of radiation exposure, the South Korean government says.
But researchers could not confirm if they were was related to Pyongyang's nuclear weapons programme.
The four are among 30 former residents of Kilju county - an area in North Korea that includes the nuclear test site Punggye-ri - who have been examined by the South Korean government since October, a month after the North conducted its sixth and most powerful nuclear test, Unification Ministry spokesman Baik Tae-hyun told a news briefing.
They were exposed to radiation between May 2009 and January 2013, and all defected to the South before the most recent test, a researcher at the Korea Atomic Energy Research Institute, which carried out the examinations, told reporters.
RELATED READING
Tillerson, Lavrov agree to continue North Korea diplomacy
North Korea has conducted six nuclear bomb tests since 2006, all in tunnels deep beneath the mountains of Punggye-ri, in defiance of UN Security Council resolutions and international condemnation.
The researcher cautioned that there were a number of ways people may be exposed to radiation, and that none of the defectors who lived had lived in Punggye-ri itself showed specific symptoms.
A series of small earthquakes in the wake of the last test - which the North claimed to be of a hydrogen bomb - prompted suspicions that it may have damaged the mountainous location in the northwest tip of the country.
Experts warned that further tests in the area could risk radioactive pollution.
After the September 3 nuclear test, China's Nuclear Safety Administration said it had begun emergency monitoring for radiation along its border with North Korea.
