The revelation follows months of confrontations between the Philippines and China in the disputed South China Sea and comes as Manila prepares for large-scale military exercises with treaty ally the United States this month.

The drones were discovered between 2022 and 2024 in locations "important strategically in the defence and the security not only of the country but for international maritime navigation", Philippine military officials told reporters at a briefing on Tuesday.

Their data collection served purposes "beyond navigation", according to Rear Admiral Roy Vincent Trinidad, who said the information could be used for "underwater warfare", detecting threats and testing weaponry below the surface.

While declining to definitively identify the drones' origin, Trinidad noted that several bore Chinese markings, while at least one had relayed a signal to China.

"Based on the technical study of the forensics of the SIM card (found on one of the five drones), the last contact of the card was in mainland China," said Trinidad, who serves as the navy's spokesman for South China Sea issues.

Three of the drones were found off the north coast of the Philippines' main island of Luzon, including two near the Balintang Channel south of Taiwan, he added.

Two others were retrieved from what were identified as "critical chokepoints", one near Masbate Island in the central Philippines and another near the southern island of Mindanao.

The Chinese Embassy did not immediately respond to requests for comment.

The Philippines separately on Tuesday said this year's Balikatan, or "shoulder to shoulder", exercises with their US counterparts would include a test of "integrated air missile defence" for the first time.

The joint exercises, which will involve approximately 10,000 soldiers, will take place from April 21 to May 9.

"We are treating exercises as rehearsals. We are implementing a plan that has been planned out in the previous Balikatan and that is what we are going to execute at this time," said Brigadier General Mike Logico.

Philippine defence chief Romeo Brawner this month told troops in northern Luzon that the island would host the bulk of the Balikatan exercises because of its strategic importance.

"These are the areas where we perceive the possibility of an attack. I do not want to sound alarmist, but we have to prepare," he said.

cgm-cwl/sco

ADMIRAL GROUP