Confirming NKorean N-test almost impossible

SEOUL, South Korea—North Korea appears all set to detonate an atomic device, but confirming the explosion when it takes place will be virtually impossible for outsiders.

Experts said Tuesday the best indication of a test will be seismic tremors and abnormal radiation in the air, but even that can be masked if North Korea wants to. In all likelihood the first word of the test will come from Pyongyang itself, just as it happened when the country conducted nuclear tests in 2006 and 2009.

Last week, North Korea warned it is planning a third nuclear test to protest toughened international sanctions meant to punish it for its long-range rocket launch in December. The world sees it as a banned ballistic missile test but Pyongyang calls it a civilian space program.