South Korea's intelligence agency said Monday that North Korea could possibly conduct additional nuclear tests, Yonhap news agency reported.
The National Intelligence Service said it detected abnormal signs at site near the place of North Korea's alleged Monday test, Yonhap reported.
The movement began at around 3 p.m. (0600 GMT) Monday of some 30-40 people and vehicles at Punggye-ri, in the northeast of the country, the report said. That had earlier been reported as a suspected underground test site.
The device that was detonated Monday was believed to be small, equivalent to less than 1 kiloton of TNT, spy chief Kim Seung-kyu said, according to Yonhap.