MANILA, Philippines — A Chinese research vessel was spotted conducting what the Philippine Coast Guard (PCG) described as unauthorized marine scientific activities well within the country’s Exclusive Economic Zone (EEZ) off the coast of Ilocos Norte.

The vessel, identified as Tan Suo 3, was detected on May 1 roughly 92 nautical miles off Burgos town, Ilocos Norte. On Monday, PCG deployed the BRP Teresa Magbanua and a patrol aircraft from Laoag to confront and escort the ship out of Philippine waters.

According to West Philippine Sea spokesperson Jay Tarriela, the crew of Tan Suo 3 did not respond to repeated radio challenges from the PCG. Instead, they were seen retrieving a deep-sea submersible called SHENHAI YONGSHI(Deep-Sea Warrior), which can dive up to 4,500 meters, and collecting an unidentified yellow object using a Rigid Hull Inflatable Boat (RHIB).

“These actions clearly indicate that the People’s Republic of China is conducting unauthorized marine scientific research activities within the Philippines’ EEZ,” Tarriela said.

The PCG aircraft conducted a maritime domain awareness flight while the coast guard ship approached the Chinese vessel. Both assets issued warnings and informed the Chinese crew that they were operating illegally within Philippine waters, citing the Philippine Maritime Zones Act, UNCLOS, and the 2016 Arbitral Award that reaffirmed the country’s sovereign rights.

PCG personnel reported that the vessel’s movements were irregular and consistent with research activity rather than innocent passage, which is allowed under international law.

The PCG said it directed the Chinese crew to stop their operations, clarify their purpose, and continue transiting through the area in compliance with international and Philippine law.

The incident is the latest in a series of confrontations between Philippine authorities and Chinese vessels operating within the West Philippine Sea. Chinese ships have frequently been accused of conducting unauthorized research, military surveillance, or harassment of Philippine assets.

PCG Commandant Admiral Ronnie Gil Gavan said the agency would continue deploying ships and aircraft to monitor and challenge any vessels conducting activities in the EEZ without prior clearance.

As of Monday, the Tan Suo 3 was being escorted out of Philippine waters. The PCG said it would maintain presence in the area to deter further unauthorized activity.