The Korean Peninsula is witnessing verbal provocations of a scale not recorded since the period following the 2010 bombardment of Yeonpyeongdo and the sinking of the Cheonan. North Korea continues to move forward with extensive militarization and the strengthening of its armed forces, according to an analysis by Modern Diplomacy. In contrast, South Korea continues to bolster its regional relationships and maintain military capabilities to ensure a level of deterrence against the Kim regime. By enhancing its missile and naval capabilities, North Korea has spent the past year upgrading its new nuclear-powered submarine, which is being equipped to launch long-range ballistic missiles. Pyongyang's pursuit of nuclear-powered submarines carries consequences not only for Seoul but also for Washington and Tokyo, as any increased capability could offer North Korea a potential amphibious first-strike nuclear package in the Indo-Pacific.
North Korea's nuclear submarine plans
North Korea currently possesses one of the largest submarine stockpiles globally, with an estimated count of 84 according to the Nuclear Threat Initiative. These submarines are categorized into distinct groups, including 40 coastal, 20 conventional, 21 midget submarines, 1 with an air-independent propulsion system, and 2 ballistic missile submarines. Since January 2021, Kim Jong Un announced a five-year plan to upgrade the country's capabilities in developing tactical nuclear weapons and submarines. Pyongyang's first operational ballistic missile submarine, launched in September 2023, was named "Hero Kim Kun Ok."
Initially keeping the program hidden from international geospatial monitoring, Kim Jong Un was reportedly photographed with a new nuclear-powered submarine featuring strategic guided missile capabilities on March 8, 2025, marking the first time Pyongyang revealed and publicly broadcast the upgraded program. Kim stated that the submarine is essential as a nuclear deterrent against "hostile forces," implying potential future use.
The decisive role of the Mutual Defense Pact with Russia
In mid-September 2025, South Korean intelligence services expressed concerns that Russia may have supplied North Korea with a nuclear reactor for the propulsion of the nuclear-powered submarine Hero Kim Kun Ok. According to government officials in Seoul, the Kremlin has supplied two to three submarine units within 2025. The provision of these units from Moscow to Pyongyang took place within the framework of the deployment of North Korean Army (KPA) troops to Kursk to bolster Russian forces against a major Ukrainian counter-offensive. Several thousand KPA soldiers were killed or wounded in a force deployment estimated to number approximately 10,000 men.
The nuclear propulsion units provided by Russia to North Korea will have a profound impact on Pyongyang's naval warfare capabilities. As nuclear propulsion allows submarines to convert energy for long-term operation, the Hero Kim Kun Ok can remain submerged for extended periods until the crew requires resupply.
Sense of urgency on the Korean Peninsula
If the DPRK makes the Hero Kim Kun Ok fully operational, equipped with Russian-made units and upgraded ballistic missiles, the existential threat to South Korea will become even more precarious. If Pyongyang gains the element of surprise, the Kim regime could order preemptive strikes on South Korea's critical infrastructure or on ROK and United States military bases in the region. South Korea's National Intelligence Service (NIS) has sounded the alarm that the strengthened relations between Russia and the DPRK will have far-reaching consequences for the peninsula at a time when Seoul is being tested by political instability. If or when the Hero Kim Kun Ok becomes active, a key strategy of the US-South Korea alliance will be to prevent the nuclear-powered submarine's exit into open waters, as the threat posed by the vessel takes on existential dimensions for Indo-Pacific countries.
Countermeasures
Currently, the Hero Kim Kun Ok is not in full operational readiness nor has it been deployed, which gives the United States and South Korea time to redesign countermeasures against the threat from the North. First, the focus on joint naval exercises, particularly in anti-submarine warfare and interception, should be a priority. Restricting North Korea's freedom of movement in the Yellow Sea and the Sea of Japan will help mitigate the KPA's first-strike options.
Second, the upgrade of missile defense systems and radar will be particularly important as Pyongyang continues missile tests that are internationally condemned. Intelligence gathering between the NIS and various American and Japanese intelligence agencies will be critical for monitoring the progress of North Korea's nuclear-powered submarine program.
On a diplomatic level, further sanctions should be imposed not only on North Korea but also on the Russian Federation. Moscow is undermining the United Nations Security Council and the arms embargo against North Korea, a fact that will require stronger diplomatic measures and sanctions in the technological sectors of both countries. Public statements about the expansion of the US nuclear umbrella in the Indo-Pacific, as articulated by then-Secretary of Defense Lloyd Austin in South Korea, will continue to be received positively. Despite the increasingly hostile rhetoric and actions of the Kim regime, Pyongyang, for now, would not unilaterally order the Hero Kim Kun Ok to launch a first strike on any potential target even if the submarine becomes active, especially as North Korea lacks the defensive capabilities to prevent any retaliation from the US and South Korea.
North Korea's growing naval capabilities must not be underestimated and, supplied with modernized ballistic missiles and Russian-made units, Pyongyang's submarine capabilities pose an existential threat not only to South Korea but also to Japan, the United States, and much of the Indo-Pacific region.
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