While North Korea’s nuclear missile program attracts the headlines, their deadliest weapon is one you can’t see.

Once the laughing-stock of world navies, North Korea is pushing ahead with new submarine technology that allows them to launch missiles at U.S. forces in Southeast Asia, or even at civilians in the U.S. — and their sub missiles can deliver chemical and biological warheads.

The darkening threat is laid out in detail in a new United Nations report, which finds that “Rapid technological developments have taken place over a short period, resulting in significant progress towards an operational submarine-launched ballistic missile system.”

For one, the regime developed a new solid-fueled missile that is more reliable and accurate than its older liquid-fueled models.

Liquid-fueled rockets are difficult to maintain, and the highly combustible mix has led to numerous failed and exploded launches.

The development of solid-fueled engines means North Korea can not only launch missiles with less preparation and at a higher rate of success, but the missiles themselves travel more consistently and are easier to aim.

But the greatest advancement, and greatest threat to the American mainland, is North Korea’s development of a more advanced submarine.

Previously, North Korea’s submarine fleet consisted mainly of Romeo-class vessels, a 1950s-era diesel model acquired from the former Soviet Union.

The submarine was so outdated that few navies still owned them, and those that did largely used them only as training vessels.

The subs were so obsolete and difficult to maintain, they frequently sank, rendering the submarine program largely useless.

Recognizing that a more advanced submarine would not only project naval force but also allow for more sudden missile attacks, Pyongyang developed the new Sinpo-class submarine.

The Sinpo-class submarine is still a diesel-fueled vessel, and not as advanced as those of other nations, but it marks a leap forward in North Korean submarine technology and gives the regime a far more potent weapon.

The goal of the new submarine program can be seen in North Korea’s propaganda films, namely one in which a missile launched from a submarine delivers a nuclear warhead directly into Washington, D.C.

With a range of only 1,500 miles, such an attack does not appear possible at the moment.

Additionally, while North Korea possesses as many as 30 nuclear warheads, none are small enough to be fitted atop any missile.

The regime is, however, feverishly working to develop such a weapon.  Afterall, they do possess between 2,500 and 5,000 tons of chemical and biological weapons, which can be delivered by missile.

The Sinpo-class submarine can successfully fire one to two Pukkuksong-1 missiles, which appears to have a range of 750 miles.

And while a diesel-fueled submarine is generally not stealthy, American naval vessels are still at tremendous risk of a deadly attack.

Development of Sinpo-class subs also gives North Korea the expertise and launching point to more quickly develop submarines capable of operating in America’s waters.

That capability, combined with the new solid-fueled missile engines and a more advanced submarine, gives North Korea the capability to launch a chemical or biological attack on South Korea and Japan, as well as against American forces stationed in the area – and eventually the American mainland.