The Philippine Coast Guard (PCG) deployed its largest vessel and an aircraft on Thursday to challenge a Chinese research ship it said was engaged in unauthorized marine scientific research (MSR) within the country's exclusive economic zone (EEZ) off Batanes province, near Taiwan.

A China Coast Guard vessel also intervened, PCG spokesman Jay Tristan Tarriela said in a Facebook post.

The patrol ship BRP Teresa Magbanua approached the Chinese research vessel JIA HAI KE 7 at about 7:55 a.m. and launched a rigid-hull inflatable boat to disrupt its activities and compel it to leave the area, Mr. Tarriela said.

JIA HAI KE 7 had been operating roughly 37 nautical miles (68.5 kilometers) northwest of Itbayat, Batanes, since the PCG confirmed its presence there on Sept. 28. Batanes, the Philippines' northernmost province, faces Taiwan across the Luzon Strait, the waterway that connects the South China Sea to the Pacific.

"JIA HAI KE 7 repeatedly maneuvered to evade the PCG units," Mr. Tarriela said, adding that the Philippine patrol vessel issued repeated radio challenges while its smaller boat maneuvered around the research ship several times.

The standoff escalated when China Coast Guard vessel CCG-2503 arrived and approached BRP Teresa Magbanua, directing repeated radio challenges at the Philippine ship. BRP Teresa Magbanua then shadowed CCG-2503 from a distance of about 1,000 yards while continuing to monitor the research vessel.

Mr. Tarriela questioned the Chinese Coast Guard's decision to intervene, noting that JIA HAI KE 7 is privately owned.

"A government law enforcement ship moving to shield a supposedly private research vessel conducting illegal MSR in another country's EEZ raises serious questions about who JIA HAI KE 7 is really working for," he said.

"It also raises questions about what data the vessel has collected over the country's undersea cable corridors in the Luzon Strait over the past four days," he added. Submarine cables carry the vast majority of intercontinental data and communications traffic, making the seabed along such corridors critical infrastructure for the countries they link.

According to Mr. Tarriela, the research ship's prolonged presence was inconsistent with ordinary passage and indicated that it was conducting marine scientific research.

In support of the operation, the PCG deployed Islander aircraft 251 from Basco Airport for an aerial surveillance flight over the area, during which the plane made several low-altitude passes, Mr. Tarriela said.

As of 11 a.m. on Thursday, BRP Teresa Magbanua remained about 37 nautical miles northwest of Itbayat, circling JIA HAI KE 7 at a distance of about 500 yards. The PCG said it would continue challenging the research vessel and maintain its presence until the Chinese ship leaves the Philippine EEZ, making the vessel's movements — and whether Beijing addresses the incident through its embassy in — the immediate points to watch.

Mr. Tarriela noted that the 1982 United Nations Convention on the Law of the Sea requires foreign entities to obtain the consent of a coastal state before conducting marine scientific research within its EEZ, a zone that extends up to 200 nautical miles from a country's baselines and places the vessel's position well inside the Philippine zone. The Philippine Maritime Zones Act likewise requires foreign entities to secure authorization before undertaking such research within the country's maritime zones, he added.

The Department of Foreign Affairs confirmed that the government had not granted JIA HAI KE 7 permission to conduct research in the area, according to the PCG.

The incident comes amid continued Philippine monitoring of Chinese vessels operating within the country's maritime zones. The Chinese Embassy in Manila did not immediately reply to a Viber message seeking comment.

In a separate development, a five-man PCG team on Sept. 28 drove away a Chinese maritime militia vessel and 24 smaller boats allegedly fishing near Kota Island in the Spratly Islands, the agency said on Tuesday. Kota Island lies within the Spratlys, an island group in the South China Sea subject to overlapping claims by China, the Philippines, and several neighboring states.

PCG personnel stationed on Kota Island, also known as Loaita Island, spotted Chinese maritime militia vessel 9622 about 1.2 nautical miles northwest of the island as it deployed 24 small boats, or sampans, each carrying two people. The boats spread out at intervals of about 50 yards and were later observed fishing within 300 yards of the island, according to the PCG.

A five-man team from Coast Guard Station Kota set out aboard a rubber boat at about 11 a.m. to challenge the vessels. The Chinese vessel ignored radio challenges, although its crew acknowledged instructions to leave through hand signals, the agency said.

— Pexcel John Bacon