Tensions have flared again in the Yellow Sea after Chinese coast guard vessels blocked a South Korean survey ship from inspecting Chinese-built maritime structures in the Provisional Measures Zone late last month.

The Center for Strategic and International Studies said in a report released Monday that China’s actions mirrored a similar confrontation in February and described them as part of Beijing’s strategy to assert de facto control over disputed waters without direct military confrontation—a “gray-zone tactic” reminiscent of Chinese behavior in the South and East China Seas.

CSIS analysis of tracking data showed that on September 24, South Korea’s research vessel Onnuri entered the Provisional Measures Zone and was soon shadowed by several Chinese coast guard ships, which later surrounded the vessel as it approached Chinese-built platforms known as Shenlan 1 and 2.

The South Korean foreign ministry acknowledged Chinese vessel movements but said there was no direct interference, adding that the Onnuri completed its mission “without incident.”

The structures, which China claims are deep-sea aquaculture facilities, have raised the alarm in Seoul, with experts suggesting they could have strategic or military uses given their proximity to Camp Humphreys, a major U.S. base in Pyeongtaek, Gyeonggi Province.

Foreign Minister Cho Hyun told lawmakers that the United States was also closely monitoring the issue, indicating growing trilateral concern about China’s efforts to expand its presence in the Yellow Sea.