Japanese Prime Minister Shinzo Abe has said that if North Korea conducted a second nuclear test, Japan and the international community would take tougher measures.
But Abe, speaking to Reuters News Agency, said it was important to make efforts to resume six-party talks on North Korea's nuclear programme as soon as possible.
South Korean officials said that activity had been spotted near a suspected nuclear test site in North Korea, but that there was no evidence to suggest Pyongyang was about to test another atomic device.
ABC News reported that Pyongyang appeared to have made preparations for a new nuclear test to follow one conducted on 9 October, a move that drew swift world condemnation and punitive UN sanctions.
But US officials also said they had no evidence suggesting that a test was imminent.
Japan banned all imports from North Korea and prohibited North Korean ships from entering Japanese ports after Pyongyang fired a barrage of test missiles in July.
Tokyo stopped exports of luxury goods in November and has been urging other nations to implement the UN sanctions imposed on the impoverished state.
