On Thursday, 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University's School of Advanced International Studies, said that satellite imagery of the Punggye-ri nuclear test site in the northeast shows that substantial tunnel excavation is continuing at the North Portal, where the last four of the five underground nuclear tests by the North were conducted. The test, if made, will be 14 times more powerful than the previous test, it said.

As grounds for his claim, Thae, who defected to Seoul in August, pointed out that the test site is located along a path linking Pyongyang and North Hamkyong Province.

"All northward roads and railways pass by Punggye-ri. If a massive explosion pollutes the area, and subsequently Pyongyang loses its control over the border areas of North Hamkyong Province, a massive defection will take place there," he said.

The environmental pollution resulting from a failure to control a nuclear explosion in a small country like North Korea will almost be an apocalypse, he said.

The international community should be aware of the danger of the North's large-scale nuclear test this time, he said.

"In particular, Punggye-ri is not far from China. Efforts to get China to help prevent the North from conducting a nuclear test should also be made," he said. (Yonhap)