It later emerged that a standoff occurred between a Korean vessel and Chinese coast guard ships in the West Sea provisional measures zone (PMZ). The Korean vessel was found to have been inspecting a structure that China had installed without authorization.
The Center for Strategic and International Studies (CSIS) said on the 27th (local time) in a report titled "A China-Korea standoff in the provisional measures zone" that "tensions between China and Korea flared again around the provisional measures zone in late September."
According to CSIS's analysis of automatic identification system (AIS) data from the maritime intelligence company Starboard Maritime Intelligence, on Sept. 24 the Onnuri, an ocean survey vessel under an agency of the Ministry of Oceans and Fisheries, entered the West Sea provisional measures zone.
The provisional measures zone is a sea area established where the two countries' 200-nautical-mile (about 370 kilometers) exclusive economic zones (EEZ) overlap, when China and Korea concluded the China-Korea Fisheries Agreement in 2000 to manage fisheries disputes. China, calling them deep-sea salmon aquaculture facilities, installed Sunran No. 1 (2018) and No. 2 (2024) in the PMZ, and in 2022 also installed a structure in the form of an oil drilling rig, saying it was a management facility.
About six hours after the Onnuri entered the provisional measures zone, a Chinese coast guard patrol ship approached the Onnuri, followed by the deployment of two additional Chinese coast guard vessels. Korea Coast Guard vessels also moved into the area to support the Onnuri.
The next day, the Onnuri and Korea Coast Guard vessels approached China's aquaculture structures Sunran No. 1 and No. 2 installed in the provisional measures zone. When the Onnuri moved toward the structures to inspect the facilities, two Chinese coast guard vessels encircled the Onnuri from both sides. The Chinese side reportedly pursued the Onnuri, which was returning to port after passing near the structures, and the Korea Coast Guard vessels for 15 hours. The distance between the two countries' vessels was said to be as little as 3 kilometers at the closest point.
CSIS said, "This incident appears similar to the standoff that occurred in Feb. 2025," and explained, "It shows a pattern of deliberately asserting presence and continuing surveillance activities around maritime structures unilaterally installed by China in disputed waters." CSIS also pointed out that this clearly violates the United Nations Convention on the Law of the Sea (UNCLOS), which guarantees freedom of navigation within the exclusive economic zone (EEZ) for all foreign vessels.
In Korea, some have argued that China's installation of structures within the provisional measures zone may be aimed at expanding its influence in the West Sea and creating grounds for sovereignty claims. The Korean government has also continued to express concern.