China is combining militia vessels with coast guard ships to expand "battlefield preparation" around Taiwan and into the western Pacific, Taiwan's Coast Guard said, warning the operations were also putting pressure on Japan and the Philippines.

Responding to a Reuters exclusive this month on the first known sighting of a Chinese militia boat off Taiwan's Pacific coast in mid-August, the coast guard said it had dispatched a patrol boat to drive away the vessel, which was operating alongside a Chinese coast guard ship and a research vessel.

The coast guard confirmed details in the Reuters report, identifying the suspected militia fishing boat as Minfudingyu 07902, and said it had come as close as about 2.7 nautical miles outside Taiwan's restricted waters on August 16.

China has increased such activity since June in overlapping economic waters involving Taiwan, Japan and the Philippines, the coast guard said, after Tokyo and Manila announced talks on maritime boundary delimitation in their exclusive economic zones.

EEZs extend 200 nautical miles from the coast and give a coastal state rights to exploit, conserve and manage natural resources in those waters.

"Such complex grey-zone operations not only increase the maritime surveillance and response burden on Taiwan, Japan and the Philippines, but may also create regional security pressure and affect the existing international maritime order," Taiwan's Coast Guard said in a statement to Reuters.

China's defence ministry, and the Philippines defence and foreign ministries did not immediately respond to requests for comment. Japan's coast guard declined immediate comment.

The coast guard said the vessels' coordinated activity was part of a broader pattern of Chinese "grey-zone" operations using law-enforcement, scientific and civilian cover to expand Beijing's influence at sea while remaining below the threshold of open conflict.

The Chinese operations were aimed at "assisting 'battlefield preparation'," the coast guard said, adding Taiwan's east coast was an important maritime traffic and supply route and that continued Chinese activity there could further increase security pressure on Taiwan.

Officials in Taiwan say they believe China is practising for a possible maritime blockade using overlapping military, paramilitary and civilian assets.

China claims democratically governed Taiwan as its own territory and has not ruled out the use of force to achieve its goals. Taiwan's government rejects the claim and says only the island's people can decide their future.

China has repeatedly said its operations off Taiwan's east coast are "legitimate and lawful" because they are in Chinese waters, a position Taipei's government strongly rejects.

Taiwan's coast guard said it had placed the Minfudingyu 07902 on a priority watchlist, assessing the boat showed the "characteristics" of Chinese maritime militia activity and said it would continue tracking the vessel.

Dating back to the 1950s, China's maritime militia uses fishing vessels with trained, state-supported crews to operate in disputed waters, often alongside coast guard ships around South China Sea flashpoints, helping Beijing assert maritime claims while maintaining a civilian appearance.

The surveillance report reviewed by Reuters was conducted by Taiwan's Apex Aviation, which used radar screenshots, infrared imagery and automatic-identification system data from a reconnaissance flight to document the cluster of vessels.