TAIPEI (Taiwan News) — The Coast Guard said Monday it tracked and warned off a Chinese research vessel suspected of conducting unauthorized hydrological surveys near Taiwan after monitoring the ship for five consecutive days outside restricted waters near Eluanbi.

The Chinese research vessel Tongji was spotted 29 nautical miles (53.7 km) southeast of Eluanbi, or 0.5 nautical miles outside Taiwan’s restricted waters. Coast Guard vessels observed the ship lowering cables into the water and suspected it was deploying scientific instruments for illegal survey operations.

The Coast Guard Administration’s Chenggong vessel approached the Tongji, created waves to disrupt the activity, and issued warnings by radio and LED message boards.

The Coast Guard monitored the vessel continuously for five days, preventing it from conducting survey activities and ultimately forcing it to recover its instruments. The Tongji later changed course and headed west away from Taiwan’s waters.

The Tongji was first detected at around 5 a.m. on May 7 and remained just outside Taiwan’s restricted waters while circling the country in an apparent attempt to collect hydrological data. The Coast Guard said it remained on high alert and was prepared for any contingency.

To prevent the vessel from entering Taiwan’s waters or conducting illegal survey activities, the Coast Guard deployed multiple ships from May 7 to May 11 to track its movements and escort it away.

After five consecutive days of operations, the Tongji had reached a position 51 nautical miles northwest of Pengjia Islet, or 21 nautical miles outside restricted waters, by around 6 a.m. Monday, and was continuing westward away from Taiwan.

The Coast Guard said Taiwan’s maritime sovereignty cannot be challenged and accused the Chinese vessel of disregarding international law by attempting to conduct illegal survey activities near Taiwan. It urged China to stop such actions, warning that they undermine regional peace and stability.