Defense Secretary Voltaire Gazmin said the June 4 incident off western Palawan province is the latest foreign intrusion into Philippine territorial waters, where the military has previously accused Chinese military and civilians ships of illegal incursions.

The fishermen, shaken but unharmed, immediately left the area they locally call Dalagang Bukid Shoal, about 131 miles (210 kilometers) off Palawan Province’s Balabac Island. The fishermen failed to identify the aircraft, which buzzed about 20 feet (six meters) over the tip of an antennae of their vessel, Gazmin said.

"It’s the latest intrusion, the latest violation," Gazmin told The Associated Press.

Gazmin declined to speculate on the aircraft’s identity but said most incursions into the Philippine waters in and near the Spratlys have been blamed on Chinese vessels.

The Spratlys, a chain of barren, largely uninhabited islands, reefs and banks in the South China Sea are claimed wholly by China, Taiwan and Vietnam and partly by the Philippines, Malaysia and Brunei. The islands are believed to be rich oil and natural gas and straddle .

The Philippines has accused Chinese vessels of intruding at least nine times into Philippine waters in recent months, while Vietnam says Chinese vessels have hindered its oil exploration surveys in an area 200 nautical miles off its central coast that it claims as its economic exclusive zone.

China says it has sovereign rights over the South China Sea, but the reported Chinese operations in the area have set off protests.

About a dozen protesters burned two Chinese flags near the U.S. Embassy on Monday, urging Washington to back its ally Philippines amid its recent spats with China over the Spratlys.

On Sunday, dozens of Vietnamese held protests for a fifth straight week in Hanoi, waving Vietnamese flags and chanting anti-Chinese slogans and carrying signs that read: "China stop lying, stop violating, stop invading."

Among the most serious incident reported by the Philippines was an alleged firing by a Chinese navy vessel on Feb. 25 to scare away Filipino fishermen from the Jackson Atoll, also near the Spratlys. Chinese Ambassador to Manila Liu Jianchao has denied Chinese forces fired at the Filipino fishermen.

He has acknowledged, however, the involvement of Chinese forces in an incident last March, when Philippine authorities accused two Chinese patrol ships of threatening to ram a Filipino oil exploration ship into leaving the Reed Bank near the Spratlys.

Liu said the Chinese forces were exercising Beijing’s sovereign rights at the Reed Bank, but the Philippine government countered that the area was within the country’s 200-mile exclusive economic zone and was never a part of the Spratlys.