Philippine Defence Secretary Voltaire Gazmin has accused China of preparing to build a structure on an uninhabited group of reefs within its territory.
The Philippine armed forces took aerial photographs of three Chinese coastguard ships and about 30 concrete blocks on the Scarborough Shoal, which China calls Huangyan Island, on August 31, Defence department spokesman Peter Galvez said.
Briefing parliament about the incident, Gazmin said the blocks were a "prelude to construction" and added that the move contravenes a 2002 declaration between China and the Association of Southeast Asian Nations to refrain from occupying land in the South China Sea.
"First rocks, then a piledriver, then a foundation," Gazmin said after his testimony. "When you get back again, if you don't survey, there will be a garrison."
The accusation adds to tensions days after President Benigno Aquino rejected conditions China set for him to attend a trade fair. The Philippines, a US treaty ally, lacks the military force to deter China from controlling disputed waters that may contain oil and gas reserves.
The Philippines asked the UN in January to rule on its maritime disputes with China, a move that leaders in Beijing oppose.
"The important thing is we put men there, so this can be prevented," Gazmin said. "We don't have the capability to do that at the moment."
Asked about Gazmin's remarks, China's Foreign Ministry spokesman Hong Lei said yesterday that he had no information about the matter.
China has accused the Philippines of illegally occupying Ayungin Shoal, where Philippine troops have been stationed since 1999.
The Philippines and Vietnam reject China's map of the sea, first published in the 1940s, as a basis for joint exploration for oil and gas.
In a blunt description of the situation, a senior judge in the Philippines said the country had lost Scarborough and would soon likely lose Ayungin, or Renai Shoal, as well.
Supreme Court Associate Justice Antonio Carpio, in a speech to the Philippine bar association on August 29, said that bringing the dispute before the international tribunal was Manila's "only viable option".
Carpio said China's claim over almost 90 per cent of the sea rested mainly on "historical rights" and its "nine-dash line". Chinese scholars were "divided" on how to assert the claim based on the nine-dash line, he said.
"China's incumbent judge in the international tribunal, Zhiguo Gao, wrote in 1994 that the nine dashes merely identify the islands owned by China within the enclosed area and do not represent a claim to all the waters and resources within the enclosed area," he said. This line had also kept changing, Carpio said.
"China's nine-dash line claim was originally represented by 11 dashes in the 1947 Chinese map, then reduced unilaterally in 1950 to nine dashes without explanation after the Communists ousted the Kuomintang from the mainland. In January this year, China released a new official map adding a 10th dash on the eastern side of Taiwan." He also said China contradicted these nine dashes when it issued in 1992 a law declaring a 12-nautical mile territorial sea from its baseline.
He described Manila's arbitration move as "an acid test" for the United Nations Convention on the Law of the Sea.
Upholding the nine-dash line means "killing UNCLOS", he said, because UNCLOS would no longer operate in the South China Sea since China will appropriate everything there, including what the entire world recognises as "the high seas".
At the China-Asean trade fair in Nanning , Ponciano Manalo, an undersecretary of the Philippine board of investments, said territorial disputes could affect trade ties. "I think territorial issues should be left to the governments and departments that are [responsible]," he said.
Additional reporting by Mimi Lau