Taipei expressed its "grave concern" to Manila yesterday after a Taiwanese fisherman was killed by armed men aboard a Philippine vessel. The incident in waters 164 nautical miles off the southernmost tip of Taiwan came as the Philippine navy intensified its patrols after Beijing reportedly sent one of its largest fishing fleets to the disputed Spratly Islands in the South China Sea. Taiwan's foreign ministry said the death occurred in overlapping economic zones of both the island and the Philippines when an armed vessel, identified as a Philippine ship, suddenly shot at a Taiwan-registered fishing boat operating in the area. The shooting caused serious damage to the boat, which subsequently lost mobility, and killed fisherman Hung Shih-cheng "The shooting caused serious damage to the boat, which subsequently lost mobility, and killed fisherman Hung Shih-cheng, 65," the ministry said in a statement. It added that the Philippine vessel was unaccounted for after the incident. The fishing boat was towed back to Taiwan by two other Taiwanese fishing vessels, under the escort of the coastguard. Three other crew members, including the skipper, were safe, the statement said. The ministry asked Taiwan's representative office in Manila to "express our grave concern to the Philippine government over the incident and demand an immediate investigation of the case", as well as to find and penalise those responsible. Taiwanese President Ma Ying-jeou had instructed all relevant government agencies to remain on top of the development of the case. Taiwanese media quoted one of the crew members as saying the armed men were aboard a Philippine naval vessel. But the ministry identified it only as a grey vessel with a white hold, which local media said matched that of a naval vessel. Yang Yi , spokesman for the Taiwan Affairs Office under the mainland State Council, yesterday condemned what he described as "a barbarian act". Though the incident did not happen anywhere in the vicinity of the highly disputed Spratly Islands, the use of firearms and an alleged Philippine naval vessel further stoked tensions in the South China Sea. A flotilla of fishing vessels in Danzhou , Hainan , reportedly set sail on Monday for the Spratly Islands, which are claimed by the mainland, Taiwan, the Philippines, Brunei, Malaysia and Vietnam. Philippine navy spokesman Colonel Edgardo Arevalo was quoted on Wednesday as saying that the Philippine navy was closely monitoring the movements of the Chinese flotilla.