MANILA, Philippines — The Philippine military recently mobilized a naval vessel and aircraft to monitor a Russian submarine as it navigated through the South China Sea near the western shore of the country last week, according to a security official on Monday.

The Russian submarine, upon receiving a radio inquiry from the Philippine navy, identified itself and indicated it was returning home to Vladivostok after participating in joint exercises with the Malaysian navy, revealed Jonathan Malaya, assistant director-general of the National Security Council.

While the submarine is permitted the right of “innocent passage” in the Philippines’ exclusive economic zone, its presence raised alarms when it was detected approximately 80 nautical miles (148 kilometers) from the coast of Mindoro on Thursday, Malaya added.

The submarine had surfaced due to adverse weather conditions, prompting its detection, according to Malaya.

In response to inquiries about the submarine, President Ferdinand Marcos Jr. expressed concerns, stating, “Any intrusion into the West Philippine Sea, of our EEZ, of our baselines is very worrisome.” He emphasized that the situation is a matter of concern for the country.

Marcos, referencing the Philippine title for the South China Sea, highlighted the ongoing tensions in the region, where the Philippines, alongside Vietnam, Malaysia, Indonesia, and other coastal nations, is grappling with an increasingly assertive China, which claims sovereignty over nearly all of the busy waterway.

A notable rise in territorial disputes, especially between China’s coast guard and naval forces and those of the Philippines, has led to enhanced monitoring efforts from the United States and other Western nations regarding this critical global trade passage.