Manila continues to fortify its military outpost in the submerged shoal, about 105 nautical miles off the coast of the western island-province of Palawan, defying China's demand to remove the rusty, World War II-era landing ship that the Philippines grounded there in 1999.
A military official said it takes less than an hour for a helicopter to fly to the shoal, making it easier for the military to bring supplies to the troops stationed there.
The sources said Manila is compelled to fortify the shoal after China launched a massive reclamation of reefs under its control in the disputed sea.
"We need to fortify the shoal because it is very strategic to our exclusive economic zone," said one official who declined to be named.
"It is for the protection of our men who are manning Ayungin shoal," said another source, using the local name for it.
Manila has been strengthening the hollowed inside of the vessel with permanent concrete structure to make the garrison more "liveable and safe" to the rotating navy and marine personnel manning the shoal.
Navy spokesman Col. Edgard Arevalo confirmed that indeed the military is "conducting minor repairs in the vessel."
"By conducting minor repairs in the vessel, the Philippine navy merely ensures that the men are not neglected," he said.
Arevalo stressed that the outpost is "a commissioned vessel with personnel on board," and "it behooves the Philippine Navy to ensure the ship's habitability and safety."
"Just as the Philippine navy is morally and duty bound to resupply its personnel with food, water, medicines, and other basic necessities, it is incumbent upon the navy leadership to ensure that the minimum survivable condition of the ship is accorded to its personnel on board," he said.
Vice Adm. Alexander Lopez, chief of the Western Command, denied that the military is constructing a "bunker" underneath the vessel, stressing that what they are doing is merely "sustaining" and "maintaining" the ship.
"It's just like our house that we have to maintain. It's different when you say 'construct' (which means) you are creating something new. So we do not use the word 'construct.' But we are maintaining so we can attend to the accommodation (needs) and the living condition of our troops," Lopez said.
"We are giving them (the troops) better accommodation," he said.
The Philippines has vowed to stand its ground at shoal, saying it "gives the Philippines a sentry advantage in stopping other countries' occupation of features nearest to the Philippines."
"We will definitely stay in that shoal because that shoal is ours and it's within our territory," another official said. "We need to sustain our presence in that shoal. We cannot allow that ship to rot."
Since February 2013, the Philippine military has noted an increase in sightings of Chinese maritime law enforcement vessels and Chinese navy ships in the vicinity of Second Thomas Shoal, part of a group of islands, rocks, reefs and cays known together as the Spratly Islands, which are claimed in part or in whole by China, Taiwan, Vietnam, Malaysia, the Philippines and Brunei.
Four claimants -- China, the Philippines, Taiwan and Vietnam -- have stationed troops in the islands they control. The Philippines has garrisoned troops on nine pieces of disputed territory, including Second Thomas Shoal.
Last year, China for the first time in 15 years blocked a Philippine navy supply mission to the shoal, raising tension between the two countries.
In May 2013, tension in the area of the shoal also rose when the Philippine military monitored the nearby presence of a Chinese warship and two Chinese maritime surveillance ships that were escorting about 30 Chinese fishing vessels.
Philippine officials said the ship was placed in the shoal "to serve as a permanent Philippine government installation in response to China's illegal occupation of Mischief Reef in 1995."
Mischief Reef, a submerged bank about 129 nautical miles off Palawan and 22 nautical miles from Second Thomas Shoal, has since then become the Chinese navy's "most active base" in the South China Sea, according to the Philippine military.
The Philippine Navy’ two Augusta Westland (AW109) Power naval helicopters at Naval Station Jose Andrada, Roxas Boulevard, Manila.