The Philippines and Vietnam are becoming involved in a potentially dangerous confrontation in the disputed Spratly Islands in the South China Sea.

The two countries - as well as Taiwan - occupy neighboring islands in the sprawling group of largely uninhabited reefs that are believed to have significant oil deposits. Both have been reinforcing their garrisons on the islands they hold. Peking also claims a portion of the island group.

One informed Philippine military officer said the United States is quietly aiding the Philippines in their military buildup in the area, but the Pentagon denied any such U.S.role.

The heart of the Philippine government's effort to strengthen its control over parts of the Spratlys is here in Puerto Princesa, capital of the Westernmost-Philippine province Palawan. A year ago, Manila organized the Western Command, called Wescom, whose mission is to establish control over the eastern portion of the Spratly Island area.

Wescom is a joint command of 3,200 Navy, Air Force, Marine, Army, and constabulary troops. Its forces are soon to be increased to 5,000.

The hurried arms buildup was spurred by the discovery of oil west of Palawan a year ago. Cities Service, drilling for the Philippine Government, struck oil in March 1976 and is drilling to determine whether oil is present in commercial quantities.

The territory in dispute is a number of small islands in the eastern portion of the Kalayaan Island group, known as the Kalayaan Islands. The disputed area is about 300 miles from Philippine shores, and slightly farther from Vietnamese shores. To further complicate matters, China and Taiwan also have claims in the area.

According to informed military sources here, the Vietnamese have recently been reingforcing their main stronghold on Pugad, one of the Spratly Islands, with anti aircraft guns and heavy coastal artillery, which were photographed by Philippine air force jets.

Last year, one of the twice daily, Philippine air patrols from Puerto Princesa was fired on by the Vietnamese garrison, which numbers about 250.

Last week, according to informal sources here, two Vietnamese supply ships and two destroyer escorts visited Pugad which is only 200 yards from the closest Philippine-occupied island. None of this has been mentioned in the Philippines'government-controlled media.

The Philippines has begun a $150 million building program to defend its claim to the area. A new 9,000-foot runway and headquarters have already been built in Puerto Princesa, and a 5,500-foot runway is nearing completion on the main contested island, Pagasa.

A long-range radar installations is planned for peak on the mountainous spine of Palawan, and a major naval base and marine training camp are being built on the west side of Palawan.

Wescom now has only a handful of antiquated American-made T-28s, which fly daily overwater patrols. It also has five U.S.-built frigates on ocean patrol and six Albatross patrol planes, four provided by the United States in 1964 and the rest in 1975.

Philippine forces include a reinforced company on Pagasa and two marine platoons spread on the other contested islands. On Palawan, three combat battalions are kept in reserve, waiting to be sent to any island that is threatened.

Philippine officers say that officers from the U.S. Military Assistance Group frequently visit here to help plan strategy and offer technical advice.

(A. Pentagon spokeman said that while U.S. officers periodically visit Philippine military commands where U.S. material is in use, they "have not played an advisory role to Wescom or any other Philippine command in recent years nor assisted in planning strategy for Philippine presence in the Spratlys.)

(U.S. rams transactions with the Philippines this fiscal year, according tho the Pentagon, include $17 million in grants, $20 million in credits and cash purchases of $70 million by the Philippines, plus transfers by the United States of some excess military items. "The United States has not provided additional assistance to the Philippines based on requirements for use in or around the Spratly Islands," the Pentagon said.)

Meanwhile, Amoco has also begun drilling for the Philippine government about 150 miles west of here. According to U.S. sources in Manila, Amoco was warned by the State Department that the area was contested by other nations and that in case of difficulties, no help could be expected from the U.S. military.

Philippine marines, however, are stationed on board the drill ships, and frigates are anchored nearby.

Informed sources say that President Ferdinand Marcos will soon announce the westward extension of the country's air defense zine to include the area where the Amoco ship is drilling.

Vietnam's air defense zone is believed to extend eastward to within less than a hundred miles from the new Philippine zone.

Oil is a key issue in the Philippines, since about a third of all import payments go for its purchase and the country is almost completely dependent on imported oil.