“The most plausible point is the structure of the pit was made so that it wasn’t a straight line that opened to the outside, but had multiple turns and also many intercepting blockades,” he said.

“We need to remember that this is deep in the mountains (where) they tested that are formed of heavy rocks, not out in flat, exposed area,” the official said, adding: “We may not find anything.”

South Korean, U.S. and European officials all noted that the trace materials sometimes decay rapidly - in the case of highly enriched uranium within a couple of days after an explosion. The longer no traces are found, the less likely that any traces will be found.

Although “there is still some time left, the chances of finding anything is getting lower and lower,” the South Korean official said.

(Additional reporting by David Chance; Editing by Marilyn W. Thompson and Eric Beech)