SEOUL, South Korea — Four defectors from the area near North Korea’s nuclear testing site showed symptoms that could be attributed to radiation exposure, but scientists said they could not conclude that the health problems had been caused by a nuclear test, the South Korean government said on Wednesday.

The four arrived in South Korea from Kilju, a county in northeastern North Korea that includes Punggye-ri, where the North has conducted all six of its nuclear tests in tunnels dug deep beneath the mountains. South Korea began conducting medical exams of defectors from that region in October, a month after the North conducted its biggest test explosion yet.

The size of that detonation on Sept. 3, which the North claimed was produced by a hydrogen bomb, raised fears of a possible escape of radioactive material into the environment.

Those fears were compounded by a series of small earthquakes reported from Kilju in recent weeks that have been attributed to underground cave-ins caused by the powerful test. Commercial satellite images have also found evidence of landslides near the test site, increasing fears of a further release of radioactive fallout if the North were to conduct another nuclear test there.