The Philippines Coast Guard said it deployed an aircraft on Tuesday to challenge a Chinese research ship that serves as a base for submersible vessels after it was spotted about 19 nautical miles off the coast of northern Cagayan province.

The pilot issued multiple radio challenges to the CRV Tan Suo Er Hao seeking to confirm whether it was conducting marine research without Manila’s consent, which would violate Philippine and international law, but received no response, the Philippine Coast Guard (PCG) said in a statement.

The 87.25‑metre deep‑sea research vessel, which left China’s Hainan province earlier this month and entered the western part of the Philippine Exclusive Economic Zone, was then monitored heading east about 55.8 nautical miles off Santa Ana, Cagayan, the PCG said.

The PCG said the Chinese research vessel was detected through Canada’s satellite-based Dark Vessel Detection system.

Cagayan, the northernmost province of Luzon near Taiwan, hosts one of nine military bases accessible to US forces under the Enhanced Defence Cooperation Agreement (EDCA).

The Chinese embassy in Manila said the vessel was on normal manoeuvres. It added that China does not accept certain Philippine domestic maritime laws, and under international law, has the right to pass through the waters as part of a maritime route.

On Tuesday, China staged war games around Taiwan, firing rockets into surrounding waters and simulating strikes and blockades in drills dubbed “Justice Mission 2025,” launched days after Washington announced a record $11.1  billion arms package for Taipei.

Philippine Defence Secretary Gilberto Teodoro said Manila was “deeply concerned” by China’s military and coast guard actions around Taiwan, saying they undermined regional peace and stability.

“This heightened scale of coercion has implications that extend beyond cross-Strait relations and into the broader Indo-Pacific community,” he said in a statement on Wednesday.