Undeterred by strengthened sanctions, "substantial" work has been going on at North Korea's nuclear test site to excavate a tunnel supporting its strongest ever detonation, according to 38 North, the website of a US institute.
At the North portal of the Punggye-ri test site, satellite imagery showed that "substantial tunnel excavation" is continuing, the website said.
The continued tunneling 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, it said. The test on September 9 last year is estimated at 15-20 kilotons yield. One kiloton is equivalent to 1,000 tons of TNT.
"Significant new tunneling activity has only been observed at the North Portal," 38 North said.
"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," it said.
In its previous test, Pyongyang claimed to have conducted a successful nuclear warhead explosion test aimed at acquiring technology for the production of diversified and miniaturized nuclear warheads to be carried by strategic missiles.
Experts said it was the strongest-ever detonation that caused an artificial earthquake of magnitude 5. South Korea's defense ministry said Pyongyang could conduct a conventional nuclear test or the underground detonation of a warhead without the use of nuclear materials.
North Korea's nuclear and long-range rocket tests triggered tough UN sanctions but leader Kim Jong-un has vowed to step up the development of atomic bombs and rockets capable of carrying nuclear warheads.
Lim Chang-won = email@example.com