How Should South Korea Respond to China's ‘Yellow Sea Project'?

The Diplomat

2 days ago

It is clear that there has been a shift in Beijing's perception of the Yellow Sea, and its new muscular stance is threatening Seoul's economic and security interests.
The Yellow Sea is a shallow body of water, about the size of California, located between the Korean Peninsula and China. It is too narrow for South Korea and China to fully claim the 200 nautical mile Exclusive Economic Zone that each is allowed under the United Nations Convention on the Law of the Sea (UNCLOS).
In 2015, the two countries began discussions aimed at establishing their maritime boundary. These negotiations are ongoing, but little progress has been publicly reported. The most recent round of talks took place in Shanghai in November 2024, but many have questioned whether China is negotiating in good faith as it keeps building new structures in the Yellow Sea.
In recent months, there has been heated discussion about Beijing's 'Yellow Sea Project,' which many Koreans see as threatening South Korea's economic and security interests. These concerns are driven by China's history of coercive behavior and unilateral maritime expansion, and also renewed doubts about the U.S. commitment to its allies since the reelection of President Donald Trump.
China's Yellow Sea Project
Chinese President Xi Jinping has sought to make China a peer competitor for the United States, looking to erode and ultimately replace the U.S.-led international order with a new framework more conducive to Chinese interests. During the imperial era, China's sphere of influence extended across the whole of East and Southeast Asia, and Beijing is keen to restore such dominance. Toward that end, China's forces – its navy, air force, coast guard, and maritime militia – now routinely deploy high-powered water cannons, physical intimidation, and other harsh tactics to advance China's claims in the South China Sea, the East China Sea, and near Taiwan.
In the Yellow Sea, the North Sea Fleet of the People's Liberation Army (PLA), which operates under the Northern Theater Command, has already expanded its operational reach to waters east of the 124-degree-east longitude line, which has long functioned as the de facto maritime boundary between China and South Korea. This is fueling concerns that China's Yellow Sea Project is fundamentally about military expansion.
Although Chinese illegal fishing activities in the Yellow Sea have been common east of 124 degrees east, especially near the Northern Limit Line (NLL) between the two Koreas, Chinese military activities have not – until recently. The People's Liberation Army Navy (PLAN) has gradually increased its operations in the Yellow Sea, aligning with China's broader anti-access/area-denial (A2/AD) strategy. This seeks to constrain combined naval operations between South Korea and the United States, limiting them to freedom of navigation and overflight.
China is apparently attempting to convert the Yellow Sea into Chinese internal waters, having already claimed the Bohai Sea (the innermost gulf of the Yellow Sea) as an 'inland sea' since 1958. This clearly disregards the spirit and the principles of UNCLOS, which came into effect in 1997, and the further extension of such claims over the whole of the Yellow Sea should be strongly resisted.
Legal Status of the Yellow Sea
Unlike the South China Sea, where sovereignty disputes and strategic posturing have routinely escalated into diplomatic crises, the Yellow Sea remains a relatively well-managed maritime space, legally. While the overlapping EEZs between South Korea and China have so far prevented formal maritime boundary delimitation, both countries have worked around the legal limbo. For example, under the bilateral fisheries agreement signed on June 30, 2001, a Provisional Measures Zone (PMZ) was established to allow for joint management of fisheries, pending a final boundary settlement.
Overt naval clashes in the Yellow Sea between South Korea and China have largely been avoided. Instead, tensions have primarily manifested in two forms: North Korean provocations near the NLL, and frequent incursions by Chinese fishing fleets into sensitive maritime areas. To date, these incidents have rarely escalated into direct confrontations between the Republic of Korea Navy (ROKN) and the PLAN.
The relative stability of the Yellow Sea is supported by hotline agreements between naval units and between maritime law enforcement agencies, and also by multilateral mechanisms to avoid unexpected contingencies, such as the Code for Unplanned Encounters at Sea.
Strategic and Operational Significance of the Yellow Sea
The Yellow Sea holds immense strategic importance for the PLAN. The majority of China's submarines, both conventional and nuclear-powered, are built and launched from the Huludao and Dalian shipyards located in the Bohai Sea. These yards have recently been expanding rapidly to accommodate China's next-generation submarine programs.
The PLAN's North Sea Fleet, headquartered in Qingdao, is tasked with the operation of the Type 001 Liaoning aircraft carrier, China's first. It was originally designated for training purposes at its commissioning in 2012, and subsequently reassigned to full Carrier Strike Group capabilities in 2014. Since then, the North Sea Fleet has been training in the Bohai Sea, including flight deck take-off and landing operations with the Shenyang J-15, an indigenous fourth-generation, twin-engine strike fighter.
The North Sea Fleet is responsible for the building, testing, training, and operating of China's strategic nuclear submarines (SSBNs). It oversees the deployment of the Type 094 Jin-class SSBNs, which carry JL-2 and the new JL-3 submarine-launched ballistic missiles. It will also soon receive China's most advanced surface warships: Type 055 Renhai-class guided missile destroyers, and Type 054B new-generation frigates, both of them stealth vessels.
China's operational behavior in the Yellow Sea also reflects its growing discomfort with South Korea-U.S. joint exercises. After the ROKS Cheonan, a naval corvette, was sunk by a torpedo from a North Korean midget submarine in 2012, South Korea-U.S. combined naval exercises were planned for the Yellow Sea. Strong Chinese opposition led to a temporary relocation of the allied exercises to the East Sea (Sea of Japan), only to return later that summer to the southern waters of the Yellow Sea. In response, the PLAN conducted a large-scale live-fire exercise in the Yellow Sea, an early manifestation of its A2/AD doctrine.
Increasingly, China's North Sea Fleet has been deployed in missions and roles to protect the strategic importance of the Yellow Sea. Navigational harassment and coercive operations target the ROKN, the Korea Coast Guard (KCG), and South Korean scientific research vessels. China also frequently declares temporary no-sail zones under the guise of maritime law enforcement or research activities.
Taken together with China's stiff resistance to South Korea-U.S. combined naval exercises, as well as its increased security and military coordination with the Russian Pacific Fleet, it is clear that there has been a shift in China's perception of the Yellow Sea. It is not only central to China's active maritime defense strategy, but also the military front line to counter U.S. Navy operations.
China has escalated its 'gray zone' tactics in the Yellow Sea, and is increasingly blurring the lines between traditional military exercises and deliberate strategic messaging, with the Northern Branch of the China Coast Guard (CCG) assuming quasi-military roles in support of naval objectives.
Since 2014, Beijing has installed a number of unmanned buoys and semi-permanent maritime structures under the pretext of oceanographic research. Many of these are situated just west of the 124-degree-east meridian.
More provocatively, between 2018 and 2022, China erected two large-scale installations within the PMZ, ostensibly for aquaculture development, without consulting South Korea. Then in 2025, a steel framework over 50 meters in both diameter and height was observed in the PMZ, with more reportedly planned. All these structures are claimed to be fishing facilities, though some experts believe that they may be disguised oil rigs. Regardless, these actions constitute a violation of the 2001 Fisheries Agreement between South Korea and China.
Strategic Implications for South Korea's Maritime Posture
From a legal standpoint, China's operations in international waters do not explicitly violate UNCLOS, but they do represent a serious threat to South Korea's operational freedom in adjacent waters. Consistent maritime operational policies are necessary, since China appears to be calibrating the aggressiveness of its maritime operations according to the perceived assertiveness/passivity of South Korea's government. Administrations should seek to maintain a consistent national policy that guides Yellow Sea operations, regardless of political leadership transitions.
Internationalizing the Yellow Sea boundary issue could be counterproductive: exaggerating China's intentions could complicate negotiations and jeopardize previous gains in bilateral maritime cooperation. Handling South Korea's concerns directly with China will serve to preserve the integrity of existing bilateral arrangements in the Yellow Sea.
To that end, the ROKN should continue to conduct appropriate freedom of navigation and overflight operations west of 124 degrees east in the Yellow Sea. Beyond simply signaling South Korea's insistence on preserving its access rights under UNCLOS, such missions must be understood as essential to deterring North Korean infiltration. Existing bilateral mechanisms, such as the hotline systems between China and South Korea, should be adequate to convey such messages.
South Korea's intelligence, surveillance, and reconnaissance (ISR) capabilities in the Yellow Sea also need to be enhanced, both for deterrence and for situational awareness. The ROKN has been relying on its obsolescent P-3C Orion aircraft for both ISR and anti-submarine warfare targeting North Korea. It has now acquired P-8A Poseidon next-generation maritime surveillance aircraft, with greatly superior operational capabilities. As well as North Korea, ISR operations should be expanded to target PLAN and PLA Air Force movements in the Yellow Sea, particularly east of the 124-degree-east line.
South Korea's maritime law enforcement also needs to update its institutional structure. Unlike the CCG, which reports directly to the Central Military Commission and operates under military-style directives, the KCG is under civilian control, so it is restricted to law enforcement and lacks the legal mandate or logistical infrastructure to engage in paramilitary contingencies. In February 2025, the Korea Institute of Ocean Science and Technology sent a maritime research vessel to investigate a large Chinese installation on the seabed of the Yellow Sea. The operation was obstructed by PLAN-affiliated CCG vessels including speed boats, but only one KCG vessel was dispatched in response.
For South Korea, the significance of the Yellow Sea is nothing less than existential. The South Korean capital, Seoul, is connected to the Yellow Sea via the Han River, which flows through the heart of the city. Two of South Korea's three naval commands are located on the Yellow Sea, and the largest U.S. overseas garrison, Camp Humphreys, is within 10 miles of its shores. The Yellow Sea has now become the focus of concentrated Chinese military and paramilitary activity, and is rapidly emerging as a theater of geostrategic competition, joining the South China Sea, the Taiwan Strait, and the East China Sea. For South Korea, this shift requires a reorientation in both strategic posture and operational readiness.
Conclusion
The recent revelation of a large Chinese structure in the Yellow Sea is the latest manifestation of China's relentless efforts to shift the strategic balance in its favor, and to erode the military effectiveness of the U.S. and its allies. South Korea must do everything it can to resist China's Yellow Sea project, otherwise the strategic balance in the region will ultimately shift in China's favor, excluding South Korea and the U.S. from most of the Yellow Sea. Then Kyushu, Okinawa and ultimately Taiwan would be vulnerable, triggering a domino effect and laying a solid foundation for further PLA expansion in the Western Pacific.