Five US officials told NBC Newsthat North Korea has increased its production of enriched uranium for nuclear weapons.

While missile and nuclear tests have halted, one official said, “there’s no evidence that they are decreasing stockpiles, or that they have stopped their production.”

North Korea has one of the world’s largest navies by number of ships. South Korea’s defense ministry estimatesPyongyang has 430 surface vessels and about 70 submarines.

Many of those subs are thought to be obsolete, but that fleet includes one Gorae-class ballistic-missile sub, which was outfitted with a new missile-launch tube in summer 2017, according to The Diplomat. (South Korea is reportedly looking to buy six US-made P-8 Poseidons, one of the world’s most advanced sub-hunting aircraft.)

The sub under construction at Sinpo may be a successor to that Gorae-class boat, advancing a program that US officials consider a threat because it could allow North Korea to achieve greater surprise for a nuclear strike.

“It’s too early to say if the North Koreans have defaulted on the Singapore agreement to denuclearize,” Yang Uk, chief defense analyst at Seoul-based private think thank the Korea Defense and Security Forum, told The Journal.

ADVERTISEMENT

“But earlier satellite images have already shown enough evidence proving North Korea has not abandoned its SLBM program,” he added, referring to submarine-launched ballistic missiles.