MANILA — The Philippine Coast Guard reported that more than 40 Chinese vessels were monitored operating near the Kalayaan Island Group in the West Philippine Sea.

In a report it said that in a statement, the PCG said its Islander 4177 aircraft spotted the Chinese oceanographic research vessel Xiang Yang Hong 33 about 7.34 nautical miles west of Rozul Reef on Wednesday, allegedly conducting unauthorized marine scientific research operations.

The agency said the vessel deployed a service boat toward the reef, while a Chinese Coast Guard vessel, CCGV BN 5309, was observed nearby. Authorities also monitored 13 Chinese maritime militia vessels anchored in the area.

According to the PCG, Xiang Yang Hong 33 departed China on April 15 and has since conducted survey activities in multiple features within the Philippines’ exclusive economic zone.

The vessel was reportedly tracked at Ayungin Shoal from April 20 to 22, Escoda Shoal from April 22 to 28, near Arellano Reef and Bulig Shoal from April 28 to May 1, inside Panganiban Reef from May 1 to 3, and at Quirino Atoll and Rozul Reef from May 3 to 6.

The PCG added that the vessel was also tracked via Automatic Identification System signals moving from Jacinto Reef to Diego Silang Reef since May 6.

During the same air patrol, the PCG said 28 additional Chinese maritime militia vessels were seen anchored near Cay 1 and Cay 2 while approaching Pag-asa Island. A Chinese Coast Guard ship, CCG 5101, was also observed about two nautical miles west of the island.

The PCG said the reported activities violate the United Nations Convention on the Law of the Sea (UNCLOS), which requires foreign states to secure consent from a coastal state before conducting marine scientific research within its exclusive economic zone.

“The Philippines has not granted such consent to the PRC for any MSR activities in these waters,” the PCG said.

PCG Commandant Admiral Ronnie Gil Gavan said the agency will continue conducting maritime domain awareness operations in line with international and domestic laws.

“We will continue to conduct lawful MDA operations and will continue to challenge any illegal activities that undermine our sovereignty and sovereign rights,” Gavan said.

Tensions in the West Philippine Sea persist as China continues to assert claims over nearly the entire South China Sea, a major global shipping route.

Areas within the Philippines’ exclusive economic zone are referred to by Manila as the West Philippine Sea, covering waters west of the archipelago including the Kalayaan Island Group and Bajo de Masinloc.

In 2016, the Permanent Court of Arbitration in The Hague ruled in favor of the Philippines, declaring China’s sweeping claims in the South China Sea to have “no legal basis.” China has rejected the ruling.