The Philippine Coast Guard has begun patrolling a vast underwater plateau known as Benham Rise, east of the main island of Luzon, after several Chinese survey ships were spotted there recently, possibly looking for submarine routes.
Coast Guard spokesman Armand Balilo said an aircraft flew over parts of the 130,000 sq km Benham Rise on Sunday, more than a week after the navy sent one of its warships to patrol the area.
At least three Chinese survey ships were spotted in the area from July to December last year.
Defence Secretary Delfin Lorenzana had suggested that this might be part of surveys to test water depths for submarine routes from the South China Sea to the Pacific.
However, China's Foreign Ministry said the ships were engaged in "normal freedom of navigation and right of innocent passage".
Philippine President Rodrigo Duterte has said he sees nothing wrong with Chinese ships passing through Benham Rise, but he has instructed the navy to put up "structures" to maintain a steady presence there. Manila is also looking into renaming the area "Philippine Rise", in a bid to highlight its sovereign rights over it.
ABOUT BENHAM RISE
• At 130,000 sq km, it is larger than the Philippines' main island of Luzon (104,700 sq km).
• An underwater plateau formed out of an extinct volcanic ridge, it is said to be rich in natural gas and manganese.
• It is named after Andrew Benham, an admiral during the American civil war who, early in his career, had a stint in Asia.
• Referred to as "Kalipung-awan" (loneliness in an isolated place) by those who fish in the waters around it.
• May be renamed "Philippine Rise" by Manila.
• Has been largely unexplored.
• Sits 250km east of Luzon, on the path of at least a dozen storms and typhoons that move across the Philippines each year.
• Is 2km to 5km deep.
• Marked the Philippines' first successful validation of a territorial claim under the UN Commission on the Limits of the Continental Shelf.
But a Chinese-language newspaper recently reported that one of the survey ships, identified as the 60-man Xiang Yang-Hong 03, had taken unspecified seabed sediments and deep-water samples when it criss-crossed Benham Rise for months as part of a "national specific task".
Security officials said the ship was also spotted in waters off Samar and Siargao islands in the central Philippines, apparently in search of other routes for Chinese submarines heading to the Western Pacific from the South China Sea.
Philippine President Rodrigo Duterte has said he sees nothing wrong with Chinese ships passing through Benham Rise, but he has instructed the navy to put up "structures" to maintain a steady presence there.
Manila is also looking into renaming the area "Philippine Rise", in a bid to highlight its sovereign rights over it.
The United Nations Commission on the Limits of the Continental Shelf confirmed in 2012 that Benham Rise is part of the Philippines' continental shelf.
These largely unexplored waters are believed to be rich in natural gas and other resources.
Analysts warn that Chinese forays into Benham Rise could be part of Beijing's larger effort to challenge US dominance in the Pacific.
Rows between Manila and Beijing have usually been about the South China Sea, west of the Philippines, a conduit for about US$5 trillion (S$7 trillion) of shipped goods annually. China lays claim to almost the entire South China Sea.
Diplomatic sources had earlier said that the Philippines and China could be looking into a "code of conduct" of their own covering not just the South China Sea but also Benham Rise.