(ATTN: ADDS U.S. analyst comment and background in last two paras)
SEOUL, March 11 (Yonhap) -- North Korea is continuing to excavate a tunnel at its nuclear test site, which may support an explosion up to 14 times more powerful than its last test, according to U.S. analysts Saturday.
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, according to the analysis posted at 38 North, a website run by the U.S.-Korea Institute at Johns Hopkins University's School of Advanced International Studies.
"The continued tunneling under Mt. Mantap via the North Portal has the potential for allowing North Korea to support additional underground nuclear tests of significantly higher explosive yields, perhaps up to 282 kilotons (or just above a quarter of a megaton)," said the article written by Frank Pabian, a Los Alamos National Laboratory (LANL) fellow, and David Coblentz, an earth and environmental science expert at Los Alamos National Laboratory.
The North's latest nuclear test on Sept. 9, 2016, is estimated at 15-20 kilotons yield. One kiloton is equivalent to 1,000 tons of TNT.
They said that after the test, new tunneling activity has only been observed at the North Portal while activity at other portals had either been sharply reduced or already halted.
"This suggests that the North Portal will very likely continue to be used as the primary test location, possibly because it provides the greatest amount of overburden and would likely be the most capable of containing the largest possible explosive yields, potentially up to just above a quarter of a megaton." they said.
Meanwhile, John Schilling, an engineering specialist at the U.S. Aerospace Corporation, said in an interview with Radio Free Asia on Saturday that North Korea is likely to successfully launch an intercontinental ballistic missile (ICBM) within the next two years and deploy it by 2020.
In the past decade, Pyongyang has stepped up efforts to develop a nuclear-tipped long-range ballistic missile capable of reaching the continental U.S.