The Philippine Coast Guard says a Chinese research vessel has conducted "illegal marine scientific research activities" inside the country's exclusive economic zone.



At a news conference on Tuesday, the coast guard said the vessel was detected entering the Philippines' EEZ last Thursday, off the northwestern coast of Luzon Island in the South China Sea. The research activities reportedly continued through Monday.



Philippine Coast Guard spokesperson Jay Tarriela said the Chinese vessel may have been conducting deep-sea mapping to support submarine navigation.



Video footage released by the coast guard shows what it says is a deep-sea submersible capable of diving to depths of 4,500 meters. The footage also shows what it describes as an inflatable boat recovering a piece of equipment.



The area where the Chinese vessel was spotted is near southern Taiwan and strategically important because it connects to the Western Pacific.



The coast guard said a patrol ship and aircraft were dispatched to urge the vessel to leave the EEZ, and that it is stepping up surveillance in the area.