North Korea’s Arsenal Has Grown Rapidly. Here’s What’s in It.

SEOUL — During a party congress meeting in January, Kim Jong-un, the leader of North Korea, offered an unusually detailed list of weapons that ​he said his country was developing to help counter foreign aggression.

Mr. Kim has spent much of the year putting some of those new weapons on display.

North Korea has conducted six missile tests since the congress, violating multiple United Nations resolutions that ban the country from developing ballistic missiles. This week the North said it had test-launched what it called its first hypersonic missile. The new weapon, the Hwasong-8, was a ballistic missile tipped with a hypersonic gliding warhead designed to detach midair.

The South Korean military said that the Hwasong-8 was in an early stage of development, indicating that North Korea was still years away from joining elite military powers like the United States and Russia. But the test was the latest signal that North Korea was developing missiles capable of delivering nuclear warheads that would be easier to hide and harder to intercept.

President Biden has warned of “responses” if North Korea continued to escalate tensions on the Korean Peninsula, but has not imposed fresh sanctions, even as the country routinely violates the U.N. resolutions. The United States has tried both sanctions and dialogue to persuade North Korea to ​give up its ​nuclear ​weapons programs. Neither has worked.

Instead, North Korea​ has rapidly expanded its nuclear program and modernized its missile fleet under Mr. Kim. The expansion of the arsenal is a growing threat to the United States and allies​ in the region. ​Here’s what’s in it.

There are nuclear warheads.

North Korea’s ballistic missiles can carry nuclear warheads, and the country conducted six increasingly sophisticated underground nuclear tests between 2006 and 2017. The last four of them happened under Mr. Kim.

Its last and most powerful nuclear test was conducted in September 2017, when North Korea claimed to have detonated ​a thermonuclear​, or hydrogen, bomb. ​Estimates of the device’s explosive power ranged from 50 to 300 kilotons.

A mere 100 kilotons would make the test six times as powerful as the bomb dropped on Hiroshima in 1945.​

North Korea has extracted plutonium, an atomic bomb fuel, from its Soviet-designed nuclear reactor in Yongbyon, north of Pyongyang. It also runs centrifuges to produce weapons-grade enriched uranium, another bomb fuel.

As of January 2020, North Korea had 30 to 40 nuclear warheads and could produce enough fissile material for six or seven bombs a year, according to an estimate by the Arms Control Association. Signs have emerged in recent months that North Korea may be preparing to ramp up its production of plutonium and highly enriched uranium in Yongbyon.

Although the world is preoccupied with the North’s nuclear weapons, the country has also stockpiled thousands of tons of chemical and biological​ weapons​ ​agents​ that it can deliver with its missiles​​. When Mr. Kim’s estranged half brother, Kim Jong-nam, was assassinated in Kuala Lumpur, Malaysia, in 2017, North Korea ​used the internationally banned VX nerve agent in the operation.

​Its missiles can fly longer ranges.

In 2017, North Korea made big strides in its weapons capabilities.

That year, the country fired its intermediate-range ballistic missile, Hwasong-12, over Japan and threatened an “enveloping” strike around the American territory of Guam. It also test-fired Hwasong-14 and Hwasong-15, the country’s first intercontinental ballistic missiles.

By the end of the year, Mr. Kim claimed that his country had the ability to launch a nuclear strike against the continental United States.

After 2017, Mr. Kim stopped testing nuclear weapons and long-range missiles but threatened to end his moratorium when talks with President Trump collapsed in 2019.

During a nighttime military parade last October, North Korea displayed a new, untested I.C.B.M. that looked bigger than any of the previous ones.

And at the party congress in January, Mr. Kim doubled down on his nuclear arms buildup, offering a laundry list of weapons he said he planned to develop. They included “multi-warhead” nuclear missiles, “hypersonic” missiles, land- and submarine-launched I.C.B.M.s that use solid fuel, and “ultramodern tactical nuclear weapons.”

Whether North Korea has mastered the technology needed to send an intercontinental nuclear warhead into space and then guide it back through the earth’s atmosphere to its target is still unclear. North Korea has yet to demonstrate that its warhead can survive the intense heat and friction created by re-entry.

Its weapons are getting more sophisticated.

When North Korea resumed missile tests in 2019 following the collapse of the Kim-Trump talks, the tests featured three new weapons, code-named KN-23, KN-24 and KN-25 by outside experts.

They each marked big advances in North Korea’s short-range ballistic missile program.

Unlike its older missiles that used liquid fuel, all three of the new missiles used solid fuel. The new solid-fuel weapons, mounted on mobile launchers, are easier to transport and hide and take less time to prepare. And at least two of them, KN-23 and KN-24, could perform low-altitude maneuvers, making them harder to intercept.

At a military parade in January, North Korea displayed what looked like a bigger, upgraded version of KN-23. Photos released by the North Korean media indicate that it was the newly developed tactical guided missile North Korea launched on March ​25.

The new missile was developed to be larger than KN-23 in order to carry a bigger warhead and more fuel. North Korea claimed that the missile could carry a 2.5-ton warhead. Defense Minister Suh Wook of South Korea later admitted that his military missed part of the North Korean missile’s trajectory because of its midair maneuvering.

North Korea also test-launched “long-range cruise missiles” in September. It called them a “strategic weapon,” indicating that it would arm the new missile with nuclear warheads.

Its last two missile tests showed that North Korea’s missiles were becoming harder to intercept.

In a test on Sept. 15, North Korea fired its missile from a train rolled out of a mountain tunnel. The mountainous country is dotted with thousands of underground military facilities where it could hide missiles before rolling them out for surprise attacks.

In the test this week, North Korea said its supersonic missile used “fuel ampoules,” or ready-to-load cartridges of fuel. That marked another stride in the North’s missile technology, missile experts said. The North’s Hwasong missiles, including the Hwasong-14 and Hwasong-15, use liquid fuel, which has to be pumped into a missile shortly before it is launched in a process that can take hours. The use of fuel cartridges can greatly shorten the fuel loading time and make missiles harder to target for pre-emptive strikes.

Mr. Kim said in January that his country would also build a nuclear-powered submarine to acquire the means to deliver nuclear weapons to its adversaries more stealthily.

North Korea has been testing its Pukguksong submarine-launched ballistic missiles since 2015.

During the military parades held last October and earlier this year, North Korea displayed what looked like two upgraded versions of its Pukguksong submarine-launched ballistic missiles. The country currently has only one submarine that can launch a ballistic missile but says it is building a new one with greater capabilities.

The arsenal ‘guarantees its success.’

North Korea has one of the largest standing armies in the world, with more than one million soldiers. But much of its equipment is old and obsolete, and the military lacks fuel and spare parts.

North Korea has sought to make up for its shortcomings by building nuclear weapons, which have also become its biggest bargaining tool.

At the January party congress, Mr. Kim said that his weapons program “never precludes diplomacy” but “guarantees its success.” He has also said he no longer holds any expectations for dialogue unless Washington makes an offer that satisfies his government.

“Pyongyang has been in a dizzying sprint to build an arsenal that contains the kinds of advanced capabilities you would find in the United States or Russia,” said Adam Mount, senior fellow at the Federation of American Scientists. “But for the most part, these have been single-serving demonstrations: Pyongyang tests the system once then moves onto the next one. It’s less clear whether they will complete testing or deployment of any or all of these systems.”