Manila: President Ferdinand Marcos Jr. expressed deep concern on Monday over the presence of a Russian attack submarine within the Philippines' exclusive economic zone (EEZ) in the South China Sea.

"Any intrusion into the West Philippine Sea, our EEZ, or our baselines is highly troubling," Marcos stated to reporters, referring to the area within the Philippines' maritime jurisdiction.

Navy spokesperson Roy Vincent Trinidad confirmed reports that a Russian Kilo-class submarine was detected 80 nautical miles off the coast of Occidental Mindoro on November 28. The Philippine navy frigate Jose Rizal made radio contact with the vessel, which identified itself as UFA 490. The submarine's crew explained they were waiting for improved weather conditions before continuing their journey to Vladivostok, Russia.

Philippine naval forces monitored and escorted the submarine to ensure adherence to maritime laws, Trinidad said.

The Russian embassy in Manila has not yet commented on the incident. Kilo-class submarines, known for their stealth capabilities, have been continuously modernized since their introduction in the 1980s.

The incident comes against the backdrop of strengthening ties between Russia and China, who declared a "no limits" partnership in 2022. Both nations have conducted live-fire naval drills in the South China Sea, further raising regional tensions.

Manila's relations with Beijing have also become increasingly strained over the last year due to overlapping claims in the South China Sea. Although a 2016 international tribunal invalidated China's expansive claims to the waterway, Beijing continues to reject the ruling.