MANILA - The Philippines must conduct constant surveillance and consistently ask China to leave Julian Felipe Reef in the West Philippine Sea to prevent it from taking over the largest feature in the Pagkakaisa Banks, a maritime law expert said Thursday.

"Right now, there's still an opportunity, a chance for a different outcome because the government acted early enough this time, immediately upon sighting this massive numbers of Chinese vessels. It cast a light on what was going on," Jay Batongbacal of the University of the Philippines Institute for Maritime Affairs and Law of the Sea, told ANC.

Julian Felipe Reef or Whitsun Reef is a large boomerang-shaped shallow coral reef at the northeast of Pagkakaisa Banks and Reefs or Union Reefs, a group of features under the jurisdiction of the Municipality of Kalayaan, Palawan. it is within the Philippines' exclusive economic zone and continental shelf.

But China says the reef, which it calls the Niu'e Jiao, is part of its Nansha Islands. It also rejects the landmark 2016 arbitration ruling that invalidated its sweeping claims over almost the entire South China Sea, of which the West Philippine Sea is a part.

• China says not planning permanent presence in Julian Felipe Reef

Philippines Defense Secretary Delfin Lorenzana blasted China's reiteration of its claims, saying they don't stand on solid ground. He has repeatedly urged China to withdraw the vessels.

On Wednesday, the Philippine foreign ministry filed another diplomatic protest against China, vowing to do so every day that the Chinese maritime militia vessels remain in the West Philippine Sea.

Batongbacal said this was a correct move of the Philippine government as the vessels had slowly dispersed to other parts of the sea.

• READ: 'Unsettling to see': Hundreds of Chinese vessels swarm Gaven Reef, Chigua Reef

If the government didn't sound alarm on the latest incursion, he warned of a possible repeat of China's takeover of Scarborough Shoal.

"That might happen again this time with respect to Whitsun Reef. And I believe that's why the government was very concerned when they saw this massive fleet just anchored at the reef," Batongbacal said.

Scarborough Shoal, called by Filipinos as Panatag Shoal or Bajo de Masinloc, and by the Chinese as Huangyan Island, off Zambales province north of Manila was the site of a 2012 standoff between the 2 countries.

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LAST PIECE OF THE PUZZLE

Batongbacal described Julian Felipe Reef as China's "last piece of the puzzle" before taking control of the entire Union Reefs.

"It is a very strategic area for the Chinese, especially when you consider that it's also the center, more or less, of this triangle of large military bases that they have constructed," he said.

"Around Whitsun reef, you have similar smaller Chinese artificial islands that host their radar, surveillance and other equipment. It could be an ideal place for some kind of headquarters to control all these military bases that they have in the area," he added.

• Philippines files diplomatic protest over reports 220 Chinese vessels in West PH Sea

Retired Supreme Court Senior Associate Justice Antonio Carpio had warned that the swarming of Chinese ships near the Julian Felipe Reef may be a prelude to occupation and building of naval bases, as China did on Mischief Reef in 1995.

Up to 220 vessels were first monitored at the reef on March 7. While the number has gone down to 44 during the last surveillance on March 29, several other Chinese ships were also monitored in other features in the West Philippine Sea, according to a government task force.

Chinese officials had denied allegations the ships moored at Julian Felipe Reef are part of Beijing's militia, describing those as fishing vessels taking shelter due to “rough sea conditions.” The Philippine foreign ministry said the claim is among China's "blatant falsehoods."

• READ: More US patrols in West Philippine Sea will deter Chinese incursions - Deputy Speaker
• READ: Lacson, Hontiveros say PH needs help of allies, int'l community vs Chinese incursions

President Rodrigo Duterte's chief legal counsel, Salvador Panelo, had said that China's "present territorial incursions is producing an unwelcome strain" in the relations of the two countries "and may trigger unwanted hostilities that both countries would rather not pursue."

The United States, Japan, Australia, Canada, and the United Kingdom have criticized China's incursions in the West Philippine Sea.