China's Maritime Build-Up East of Taiwan
Chinese government vessels have been sighted around Taiwan at record levels for a third consecutive month, according to Taiwan's defence ministry. In the first 24 days of August, mainland Chinese coastguard, survey, and other official vessels were detected 118 times, surpassing July's 117 sightings and June's 111. These numbers are the highest since Taiwan began publishing detection data in July 2024; before June, the maximum monthly count was 50, recorded in April 2025, according to a tally by the South China Morning Post.
The increase coincides with a period of heightened regional activity. On May 28, Japan and the Philippines announced new initiatives, including talks on sharing classified military information, the transfer of retiring destroyers, and negotiations to delimit their overlapping maritime boundaries in the Philippine Sea. Beijing condemned the delimitation talks as "illegal, null, and void," asserting that part of the area overlaps with claims extending from Taiwan. Taipei responded cautiously, stating it should eventually be consulted and that its fishing rights must be safeguarded.
The overlapping claims originate from the opposite-facing coasts of Japan's Ryukyu Islands and the Philippines' Batanes Islands, which are less than 400 nautical miles apart, producing overlapping exclusive economic zones and continental shelf claims under the United Nations Convention on the Law of the Sea (UNCLOS). This overlap does not involve the Senkaku Islands, which lie further northeast and are administered by Tokyo but also claimed by Beijing and Taipei.
Survey and Law Enforcement Operations
An August 19 report by the Asia Maritime Transparency Initiative (AMTI), a programme of the Centre for Strategic and International Studies, documented a "more sustained and assertive presence" by Chinese law-enforcement and survey vessels in waters east of Taiwan. Using commercial automatic identification system (AIS) data, AMTI showed that activity between June and August was concentrated in areas where Chinese, Japanese, and Philippine maritime claims overlap. Almost no such activity had been observed in that area between January and late May.
AMTI identified two China Coast Guard vessels as accounting for much of the activity, with a third patrolling since August 4. The research ship Xiang Yang Hong 3 joined the patrol on August 14, and the survey operation was ongoing as of publication.
China's Ministry of Transport conducted what it described as a "maritime traffic law enforcement and hydrographic survey operation," deploying the 10,000-ton patrol vessel Hai Xun 09. Additionally, the Ministry of Natural Resources carried out a separate marine environmental survey. Chinese state commentary framed these efforts as building understanding of "jurisdictional areas."
Within days of the Japan-Philippines announcement, Chinese law-enforcement vessels hailed 198 commercial ships in the area, demanding information about their ports of call and crews. AMTI noted that such demands would be unlawful even within China's exclusive economic zone. Chinese vessels also reportedly operated near submarine-cable routes.
Regional Reactions and Legal Context
Taiwanese officials warned by early July that the sustained presence appeared intended to establish a "new status quo" in the area. Australia, France, Germany, Taiwan, the United Kingdom, and the United States condemned China's maritime operations.
Most of the survey activity fell within 50 nautical miles of Taiwan, in areas largely irrelevant to the Japan-Philippines delimitation, according to AMTI. Analysts suggest the operations could function as a rehearsal for a blockade or quarantine, or to test regional reactions and normalize low-level coercion.
Under international law, UNCLOS Article 74.1 requires states with opposite or adjacent coasts to delimit their exclusive economic zones by agreement on the basis of international law to achieve an equitable solution. Article 74.4 permits compulsory dispute resolution if no agreement is reached within a reasonable period.
Legal precedent supports the notion that third-party claims do not bar delimitation between two states. In Cameroon v. Nigeria (2002) and Nicaragua v. Colombia (2012), the International Court of Justice rejected requests by neighboring states to intervene based on overlapping claims. In Bangladesh v. Myanmar (2012), the International Tribunal for the Law of the Sea rejected Myanmar's argument that India's claim prevented delimitation. Treaties and agreements between two states have no power over non-parties, and international judgements on maritime delimitation have consistently accounted for possible third-state claims.