Metro Manila (CNN Philippines, August 19) — Supreme Court Associate Justice Antonio Carpio on Saturday urged the government to protest the reported presence of Chinese ships near Pag-asa Island in the disputed South China Sea.

Carpio in a statement called on President Rodrigo Duterte and Foreign Affairs Secretary Alan Peter Cayetano to send navy ships to areas where Chinese vessels were spotted.

"The very least that they could do now is to vigorously protest this invasion of Philippine territory by China," Carpio said in a statement.

"If both are courageous, they should send a Philippine navy ship to guard Sandy Cay, and if the Chinese navy ships attack the Philippine navy vessel, they should invoke the Phil-US Mutual Defense Treaty," he added.

The Mutual Defense Treaty would compel the U.S. to extend military assistance to the Philippines if any of its territory is attacked by foreign forces.

Carpio, who has been vocal about the South China Sea dispute, said Chinese vessels are blocking Filipino fishermen in Sandy Cay, just over 2.5 nautical miles west of Pag-asa.

This disappearing sand bar, Carpio said, has become permanently above water at high tide because of China's dredging in the nearby Subi Reef.

"Sandy Cay is a Philippine land territory that is being seized (to put it mildly), or being invaded (to put it frankly), by China," he said.

Guard Sandy Cay or lose Subi Reef

Carpio warned the Philippines may lose Subi Reef if it does not guard this "new geologic feature."

"If China acquires sovereignty over Sandy Cay, it can now claim Subi Reef as part of the territorial sea of Sandy Cay, legitimizing China's claim over Subi Reef and removing Subi Reef from the continental shelf of the Philippines," Carpio said.

The arbitral ruling that awarded to the Philippines areas in the South China Sea, said the Subi Reef as a low-tide elevation does not generate entitlement to a territorial sea or an exclusive economic zone (EEZ), which can be awarded to a country or state.

But since it lies within 12 nautical miles of the reef on which Sandy Cay is located, "it could serve as a basepoint for the territorial sea of Sandy Cay," the ruling said.

If China successfully occupies Sandy Cay, Carpio said it would be tantamount to reducing by a third or more the territorial sea of Pag-asa.

"It will also prevent the Philippines from extending the territorial sea of Pagasa to include Subi Reef. By any yardstick, this is seizure of Philippine territory," he said.

He called it "worse than what happened in Scarborough Shoal," where China is reportedly planning to build environmental monitoring stations.

The government has accepted China's pronouncement it has no official plans to build anything on Scarborough, also known as Panatag Shoal.

Confirming Chinese presence

Magdalo Representative Gary Alejano on Tuesday reported his sources "inside the military" said since Aug. 12, China has deployed two frigates, one coast guard vessel, and two large fishing vessels along with militia, stationed two to five nautical miles away from Pag-asa Island.

A day later, Cayetano did not confirm or deny Alejano's claims, but said the "presence of ships alone does not mean anything."

Amid calls for the government to file a diplomatic protest, Cayetano said, "Nagrereklamo pa rin tayo [We are still protesting], it's just that we don't announce it and we don't tell you how we do it."

The Armed Forces of the Philippines (AFP) on Friday said the military's Western Command (Wescom) is verifying reports and ensuring the safety of Filipino fisherman in the area.

The Wescom is in charge of patrolling the South China Sea, including Pag-asa Island, internationally known as Thitu Island and one of the biggest islands in the disputed Spratlys.

Philippine Coast Guard Officer-In-Charge Commodore Joel Garcia said they have yet to verify Alejano's claim that a Filipino fishing vessel was prevented by Chinese fishing vessels from going near the sandbars west of Pag-asa.

Meanwhile, Washington-based think thank Asian Maritime Transparency Institute (AMTI) posted Aug. 13 photos of what it said were Chinese flotilla near Pag-asa Island.

"On that day, there were nine Chinese fishing ships and two naval/law enforcement vessels visible near Thitu, with others possibly under cloud cover," the AMTI said in its website.

"It could be claimed that these ships are all operating within the territorial sea of both Thitu and the sand cay(s) with which Subi is associated," it added.

It also reported sightings of a Philippine fishing boat docked at a sandbar, "possibly sent out from Thitu to investigate the Chinese presence."