"We have also seen the reports (of the North's possible nuclear test) and are closely monitoring the situation," Elisabeth W?chter, the global body's chief public information official, told Yonhap News Agency.

With the CTBTO's International Monitoring System (IMS), she added, "No nuclear test would go undetected."

New satellite images suggest that the North is in the final stage of preparing for a nuclear test, according to the 38 North website based in Washington, D.C.

It said the commercial satellite imagery, taken on March 25, shows the continued presence of three to four vehicles or equipment trailers at the entrance of the North Portal, an underground tunnel where the North carried out its previous nuclear tests, in the Punggye-ri site, North Hamgyeong Province.

"The texture of the ground from the portal entrance past the vehicles or equipment trailers suggests that communications cables may have been laid on the ground," it said, pointing out that the equipment would likely be used to initiate the test, collect data from the explosion and process the data.

Asked about the report, South Korean intelligence sources agreed that the North may be all set for its sixth nuclear test.

"The current scene in Punggye-ri is similar to that of final-phase preparations for North Korea's past nuclear tests," a source said. "It can be judged that all preparations are virtually over."

Experts predicted the North will focus on enhancing the explosive yield of a nuclear weapon this time.

"North Korea claims that it has already miniaturized a nuclear bomb. Thus, there is a chance that it will test a boosted fission bomb," Lee Chun-geun, a researcher at the Science and Technology Policy Institute, said. "It could be 150-200 kilotons."

The power of the North's previous nuclear test on Sept. 9, 2016, was estimated at 10 kilotons.

Lee did not rule out the possibility of the North detonating multiple nuclear bombs at a main tunnel and some side ones. (Yonhap)