MANILA: The Philippines is investigating a maritime incident to find out what killed three Filipino fishermen in a ramming incident that sank their boat in the South China Sea, its president said on Wednesday.

"We assure the victims, their families, and everyone that we will exert every effort to hold accountable those who are responsible for this unfortunate maritime incident," Ferdinand Marcos Jr said on the X social media platform.

Three Filipino fishermen died after their fishing boat was rammed by an unidentified foreign commercial vessel while crossing the South China Sea, the Philippine Coast Guard (PCG) said on Wednesday.

The incident occurred on Monday while the boat was transiting 85 nautical miles (157 km) northwest of the disputed Scarborough Shoal, it said in a statement. Eleven crew members survived when the boat sank.

"The incident is still under investigation to ascertain the details and circumstances surrounding the collision between the fishing boat and a still unidentified commercial vessel," Marcos said.

"Let us allow the PCG to do its job and investigate, and let us refrain from engaging in speculation in the meantime," he added, referring to the coast guard.

Tensions around those waters have recently flared up after the Philippines said it removed a 300-metre ball-buoy barrier installed by China's coastguard near the Scarborough Shoal, a prime fishing spot and one of Asia's most contested maritime features.

The strategic shoal, named after a British cargo vessel that ran aground there in the 18th century, is in the Philippines' exclusive economic zone but was seized in 2012 by China, which has maintained a constant presence of coast guard ships and fishing trawlers there ever since.

China has rejected the Philippines' version of events over the barrier, while the United States has weighed in with support behind Manila and vowed to honour its treaty commitments to defend its treaty ally if attacked.

The Philippine coastguard did not elaborate on the incident or provide details of the vessel it said had rammed the Filipino crew.