An aerial photo taken though a glass window of a Philippine military plane shows the alleged on-going land reclamation by China on mischief reef in the Spratly Islands in the South China Sea, west of Palawan, Philippines, May 11, 2015. Ritchie B. Tongo/Pool/File Photo via Reuters

MANILA - The Philippines should have strongly protested China's installation of weapons on artificial islands in disputed waters instead of merely sending a "weak" diplomatic note, Manila's former envoy to the United States said Wednesday.

Beijing will predictably "ignore" the note verbale, which sought "clarification" on anti-aircraft weapons it allegedly put up in the South China Sea, said former Ambassador Jose Cuisia Jr.

"It was a weak note verbale," he told ABS-CBN News. "You got to at least say, 'Hey, you can't continue putting military weapons in those islands.' That's the minimum that they could do."

Foreign Secretary Perfecto Yasay Jr. earlier said the Philippines had sent three to four diplomatic notes to China since Duterte took office in June last year.

The notes are a common form of diplomatic communication often unsigned and written in the third person.

Cuisia acknowledged China would have disregarded a formal diplomatic protest like it had done in the past over the maritime dispute with the Philippines.

"Wala tayong magawa e (There's nothing we can do)," he said. "That's why we need the international community to pressure China to respect our rights... the US and the rest of the international community."

By installing the weapons, China "violated" an existing declaration with Southeast Asia nations to exercise "self-restraint" in the area, said professor Jay Batongbacal of the University of the Philippines Institute for Maritime Affairs and Law of the Sea.

The 2002 declaration also called on parties not to occupy "presently uninhabited islands, reefs, shoals, cays, and other features and to handle their differences in a constructive manner."

"The Philippines really should have protested the installation of these weapons systems," Batongbacal told ABS-CBN News.

"At the very least, it should have strongly asserted its legal position that the weaponization of these islands, in the first place, contravenes China's commitment not to militarize them."