What do North Korea’s sub missile tests really prove?

North Korea’s recent underwater launch of strategic cruise missiles underscores its accelerating drive to field a survivable, sea-based nuclear deterrent, even as significant engineering hurdles and platform limitations continue to constrain its naval ambitions.

North Korea test-fired two types of submarine-launched strategic cruise missiles into the Sea of Japan, marking its first public underwater cruise missile test in nearly three years, according to state media and regional military officials, multiple media outlets reported this month.

Senior defense officials Kim Jong Sik and Jang Chang Ha oversaw the drill, which was intended to verify the operational readiness of naval combat systems and demonstrate the reliability of North Korea’s nuclear war deterrent, the state-run Korean Central News Agency (KCNA) reported.

Photographic evidence indicates the weapons were deployed from the 3,000-ton Hero Kim Kun Ok, a modified diesel-powered submarine unveiled in September 2023.

South Korea’s Joint Chiefs of Staff, which tracked the projectiles from launch to terminal flight, assessed that one missile was fired from a vertical launch system while the second was deployed horizontally from a torpedo tube — a staggered tactic that could complicate air defense radar detection.

Still, the test is not fully conclusive evidence that North Korea already has a functional sea-based nuclear deterrent. Test-firing a cruise missile from a submerged submarine may be a significant capability milestone, although it is only part of an entire system needed for a sea-based nuclear deterrent.

The submarine must operate reliably, maintain its weapons, avoid detection, and perform missions under realistic circumstances. Nuclear command and control capabilities – especially communicating with submerged submarines – must also be validated.

Looking at North Korea’s broader efforts to develop a survivable, sea-based nuclear capability, the country has not relied solely on undersea hulls, having previously test-fired “strategic cruise missiles” from surface warships.

In April 2026, North Korean leader Kim Jong Un oversaw the launch of two nuclear-capable strategic cruise missiles alongside three anti-ship missiles from the country’s largest 5,000-ton destroyer, the Choe Hyon, off Nampo to verify its integrated weapon command systems.

This Western coast trial followed earlier testing in August 2023, when North Korea fired purported Hwasal-2 strategic cruise missiles from an Amnok-class corvette off Munchon, demonstrating an evolving push to disperse nuclear-capable strike platforms across its surface fleet.

By distributing cruise missile launch points across both eastern and western coastal areas, North Korea clearly seeks to stretch allied tracking networks and present multi-axis strike threats during crises.

Because surface warships are relatively vulnerable to attack, these capabilities may be temporary as North Korea develops a more survivable undersea nuclear deterrent.

North Korea may be preparing to test a submarine-launched ballistic missile (SLBM), complementing its strategic cruise missiles. A US Congressional Research Service (CRS) report this month says North Korea demonstrated operational capability in 2019 by flight-testing the nuclear-capable Pukguksong-3 (KN-26) and then displayed the Pukguksong-4, -5, and -6.

However, the development of operational submarines capable of effectively deploying them has lagged behind North Korea’s naval missile technology. Hong Min mentions in a September 2023 report for the Korean Institute for National Unification (KINU) that North Korea is pursuing a dual-track naval strategy to deploy underwater nuclear strike capabilities.

According to Min, the short-term first track focuses on retrofitting approximately 20 aging, diesel-electric Romeo-class submarines with vertical launch systems (VLS) to deploy tactical nuclear-armed ballistic missiles, cruise missiles, and nuclear torpedoes—exemplified by the Hero Kim Kun Ok.

Joseph Bermudez Jr. and other authors mention in a September 2023 Beyond Parallel report that the Hero Kim Kun Ok represents a heavily modified Soviet-era Romeo-class vessel engineered to deploy SLBMs.

Bermudez and others say that the submarine features a reshaped bow, relocated diving planes for stability, and an aft-sail missile compartment housing ten vertical launch tubes—four large and six smaller cells intended for Pukguksong-series and modified KN-23 ballistic missiles.

However, Sukjoon Yoon points out in an October 2023 report for the S. Rajaratnam School of International Studies (RSIS) that the Hero Kim Kun Ok has structural weaknesses because it breaks the standard 10:1 Romeo-class length-to-width ratio, reducing hull strength for underwater launches.

Yoon explains that modifying the vessel’s original dimensions without a corresponding hull lengthening compromises structural integrity under dynamic underwater pressure.

He says an elevated center of gravity impedes submerged maneuverability, an unreliable superstructure makes cold launches risky, and added conning-tower stabilizers make the submarine considerably noisier, and easier to detect.

Given those design flaws, North Korea may be designing a follow-up to the Hero Kim Kun Ok, with Min saying that the country’s secondary long-term aim is to develop true nuclear-powered attack and ballistic missile submarines (SSN/SSBN).

In December 2025, North Korean state media released images of what appears to be the completed welded hull of an 8,700-ton “nuclear-powered strategic guided missile submarine.”

However, it is unclear how far along North Korea is on this project. As Vann H. Van Diepen points out in a March 2025 38North article, hull fabrication may have started at least a decade ago, but developing a compact nuclear reactor remains the pacing challenge, with several years of fitting out and sea trials expected before an operational vessel emerges.

Russia may be involved in North Korea’s efforts to develop nuclear propulsion, although it has not been definitively proven.

Anton Ponomarenko mentions in a May 2026 38North report that evidence centers on the dimensional compatibility between North Korea’s 8,700-ton nuclear submarine hull and Russian Akula-class OK-650 reactors, alongside the suspicious December 2024 sinking of the Russian military-linked freighter Ursa Major, which allegedly carried suspected naval nuclear reactor components destined for North Korea.

According to Ponomarenko, Russia views a nuclear-armed North Korea as a regional force multiplier and diplomatic leverage against China while securing North Korean munitions and troops for its war effort in Ukraine.

However, he says full reactor transfers remain speculative without the findings from a conclusive official investigation into the Ursa Major’s cargo manifest.

But even a retrofitted submarine like the Hero Kim Kun Ok could still pose a threat. Although it could be noisy and easily detected as soon as it leaves port, it could still launch cruise missiles from unpredictable locations and stretch South Korean, US, and Japanese anti-submarine capabilities, no matter how formidable they are.

While North Korea’s cruise missile test from the Hero Kim Kun Ok indicates its determined efforts to advance the development of a survivable sea-based deterrent, the effectiveness of such a deterrent may largely depend on its progress with subsequent submarine designs – possibly with Russian assistance.

Until North Korea can field a truly quiet, domestically proven nuclear propulsion system, its undersea fleet will remain caught between tactical ambition and profound engineering compromise.