The Philippine military deployed a navy ship and air force planes to shadow a Russian submarine, which passed through the South China Sea off the country's western coast last week, a security official said Monday.
The Russian submarine identified itself in response to a Philippine navy two-way radio inquiry, saying it was en route home to Russia's eastern city of Vladivostok after joining an exercise with the Malaysian navy, Jonathan Malaya, assistant director-general of the National Security Council, said.
The submarine, like other foreign ships, has the right of "innocent passage" in the country's exclusive economic zone but it still sparked concern when it was spotted on Thursday about 80 nautical miles (148 kilometers) off the Philippine province of Mindoro, Malaya said.
The submarine was sighted after it surfaced due to weather-related conditions, Malaya said.
"All of that is very concerning," President Ferdinand Marcos Jr. told reporters when asked about the submarine. "Any intrusion into the West Philippine Sea, of our EEZ, of our baselines is very worrisome. So, yes, it's just another one."
Marcos used the Philippine name for the South China Sea, where his country plus Vietnam, Malaysia, Indonesia and other coastal states have faced an increasingly aggressive China, which claims the busy waterway virtually in its entirety.
An alarming spike in territorial confrontations, particularly between Chinese and Philippine coast guard and naval forces, starting last year has prompted closer surveillance by the United States and other Western governments of the key global trade route.
The Philippines coast guard said Monday that a Chinese military helicopter flew close to fishing boats manned by Filipinos in a "dangerous act of harassment" last week at Iroquois Reef, a disputed fishing area in the South China Sea.
Two Philippine coast guard patrol ships have been deployed to the area to protect Filipino fishermen, coast guard spokesperson Commodore Jay Tarriela said.
There was no immediate comment from Chinese officials.