The Defense Department plans to invest $1.3 billion to upgrade Clark Air Base and Subic Bay naval base in the communist-threatened Philippines despite the conclusions of two U.S. intelligence agencies that the government of President Ferdinand Marcos is headed for "catastrophe."

Wary of the growing threat from Filipino guerrillas, however, the Pentagon has begun to look for alternative Pacific sites in what could be an $8 billion relocation of the two biggest U.S. bases outside the United States, according to a high-ranking Pentagon official.

The Central Intelligence Agency and the Defense Intelligence Agency recently told the House Intelligence and Armed Services committees in secret briefings that present trends in the Philippines are "really downward and very negative" and heading for "catastrophe," Rep. Dave McCurdy (D-Okla.), a member of both panels, said yesterday.

"The intelligence community is so far out ahead of the policy-makers that it's incredible," McCurdy added.

Reacting to a recent Senate report questioning the wisdom of a large new U.S. military investment in the Philippines, Assistant Secretary of Defense Richard L. Armitage said in an interview that "the facts as we see them are that we will be at those facilities at least until 1991."

After that, Armitage added, "it gets problematical . . . . We are looking seriously at alternatives." The potential loss of the 83-year-old bases -- and their potential conversion into Soviet facilities -- is particularly worrisome to the Pentagon because of the large Soviet naval presence at Cam Ranh Bay in Vietnam, once a major U.S. base.

Among the alternatives under consideration, Armitage said, are Guam, which lies 1,500 miles east of the Philippines, and Tinian, an island in the northern Marianas, where the United States has already leased more than 18,000 acres.

Both are regarded as poor options, however, because of their location, lack of development and manpower shortages. Also, Armitage said replacement costs could total "as much as $8 billion."

A communist insurgency led by the New People's Army (NPA) has spread virtually throughout the Philippines. The movement contends it receives no help from the Soviet Union or other communist-bloc nations; demands for removal of the U.S. bases are heard not only from the NPA but also from noncommunist opposition elements in parliament, where a 17-member committee is studying the issue.

Several ministers in the Marcos government have also called for abrogation of the military bases agreement with Washington; the agreement comes up for review in 1989 and for possible renegotiation in 1991. But these semiofficial requests may be linked to a government maneuver to obtain more than the $900 million the United States now provides to the Philippines in economic and military aid over a five-year period ending in 1989, according to congressional and administration sources.

A Soviet military presence in the Philippines could result in "a basic reversal of the strategic balance" in the region, Armitage said. Navy Secretary John F. Lehman Jr. echoed that sentiment in a recent interview, saying it "would be a horrendous situation were the Soviets able to operate out of the Philippines."

Armitage, the Pentagon's top Asia expert, said there was a "slight possibility" of looking for alternative U.S. bases closer to the Asian mainland to counter Cam Ranh Bay, where Moscow now has its largest base outside the Soviet Union. However, the Pentagon has no plans to approach any potential host governments in the Pacific region to ask for facilities, he added.

Armitage specifically denied that the Pentagon was considering Singapore as an alternative to Subic Bay, saying it was too crowded and short on storage facilities. There have been unconfirmed reports here that the United States is interested in the former British base at Woodlands in Singapore.

The mounting problems for the United States in continuing to depend on its two facilities in the Philippines were vividly brought home in a Sept. 9 report written by the minority staff of the Senate Armed Services subcommittee on military construction.

The report, based on a visit in late August, found serious security breaches at both bases, including no sign of Filipino guards, empty guard towers and only seven miles of the 26-mile outer perimeter fence still standing at Clark. At Subic Bay, the subcommittee staff learned that an armed NPA squad had recently camped inside the perimeter of the naval base, only a mile from a U.S. ammunition magazine.

"Although the magazine is secured with the proper security fencing and guarded by U.S. Marines, the fact the NPA can actually operate inside the perimeter guarded by the Philippine military is nothing short of incredible," the report said.

At both Clark and Subic Bay, the Philippine government has responsibility for securing the perimeters, and the United States protects the installations inside.

Armitage acknowledged "some real holes" in security arrangements and noted that Clark is located in a traditional home base area for the communists. The Senate report provided "some more momentum" to discussions already under way between the Defense Department and the Philippines government about improving security, he added.

Both Armitage and the subcommittee staff appeared to agree that the NPA would never be strong enough to actually occupy either base and had adopted a policy of avoiding attacks either on U.S. facilities or individual Americans. But the report said the communists could shift tactics "overnight" and destroy important, costly U.S. military equipment.

Armitage agreed that the NPA could attack the bases "any time" but doubted it would do so. U.S. military "flying squads" have been set up as a precaution to intercept intruders, he said.

The Senate report urged the Pentagon to delay its plan to spend $1.3 billion for upgraded housing, runways and other facilities "until greater certainty is established that the United States will continue to be a welcome guest."

But Armitage said he had not changed his "risk assessment" of last spring that the military struggle between the Marcos government and the NPA could end in a stalemate on the battlefield "in three to five years time" if present trends continue.

"I would state that the trend in the Philippines is still deteriorating," although at "a slightly lower rate" in recent months, he said.

He attributed this partly to "rudimentary improvements" in the performance of the Philippine armed forces and partly to spreading disenchantment with the NPA among a pro-American Filipino population.

"It has more to do with the fact that a very westward-leaning, U.S.-oriented population, which generally shares our values, is getting the word that maybe the NPA is not the nice people," he added. "Once they consolidate control and they have their apparatus in place, things become a little more draconian."