MANILA - A wrecked navy transport ship perched on a remote coral reef could be the next flashpoint in the South China Sea, where China and five other parties claim sovereignty.
Manila is accusing Beijing of encroachment after three Chinese ships, including a naval frigate, converged just five nautical miles, or 9km, from an old transport ship that the Philippines deliberately ran aground on a reef in 1999 to mark its territory.
Philippine officials say they fear the Chinese ships will block supplies to about a dozen Filipino marines stationed in abject conditions on the rusting ship.
The area, known as Second Thomas Shoal, is a strategic gateway to Reed Bank, believed to be rich in oil and natural gas.
In 2010, Manila awarded an Anglo-Filipino consortium a licence to explore for gas on Reed Bank, but drilling stalled last year due to the presence of Chinese ships.
Manila says Reed Bank is within the country's 200-nautical mile exclusive economic zone. Beijing says it is part of the Spratlys, claimed entirely by China, Taiwan and Vietnam and in part by Malaysia, Brunei and the Philippines.
"China should pull out of the area because under international law, they do not have the right to be there," said Mr Raul Hernandez, a spokesman for the Philippine Department of Foreign Affairs, noting the area's proximity to Palawan. The Chinese ships were a "provocation and illegal presence", he added.
Chinese Foreign Ministry spokesman Hong Lei said on Tuesday that the Second Thomas Shoal was part of the Spratly Islands, over which China had "indisputable sovereignty".
"It is beyond reproach for Chinese boats to carry out patrols in these waters," Mr Hong said, adding that China called on all parties to "refrain from taking actions that complicate the situation".
As of Tuesday, two Chinese marine surveillance ships remained in the area, Philippine spokesman Colonel Edgardo Arevalo said, adding that the frigate and some fishing boats had left.
Institute of Southeast Asian Studies analyst Ian Storey said tension at Second Thomas Shoal could prove more dangerous than last year's stand-off at unoccupied Scarborough Shoal, given the presence of Filipino troops.
"It is hard to imagine China using force to gain full control... but some kind of blockade to drive out the Philippines' troops would have to be a possibility," Dr Storey said. "There is a real chance of escalation or miscalculation."