There is an increasing danger that the United States and the Soviet Union could be dragged into a Third World conflict where access to natural resources is at stake, Defense Secretary Harold Brown told The Washington Post in a year-end interview.

Brown said he was worried about the general problem raised by the conflicting interests of the two super-powers in developing countries, and said, in response to a question, kthat the current turmoil in oil-rich Iran is an example.

Brown said the prospects of instability in developing countries are such "that we might have a very difficult time avoiding the choice between active participation in conflict, although not necessarily with ground forces, or a severe damage to our national interests and resources.

"I think that's a worse problem than it was in the 1950s and 1960s," Brown continued. "You say how could it be worse than Vietnam? I guess what I'm saying is that our vital interests are more likely to involved than in retrospect they probably were" in Vietnam. "We're more interdependent; we're more resource-dependent on the outside world."

Brown was part of the government team during the Vietnam war, serving under President Kennedy as Pentagon research director and later as secretary of the Air Dforce. He said he doesn't believe the United States would become involved again in a place like Vietnam where vital national interests were not unequivocably at stake.

The defense secretary, who has just finished two years of running the world's mightiest military establishment, did not spell out how the United States could successfully use its power in developing nations where vital resources were in contention. He did state, however, that "planning is going ahead" for a highly mobile, hard-hitting specialized force for conflicts outside of the North Atlantic Treaty Organization.

This time last year, Brown and other Carter administration executives were talking about assembling two Army divisions and a Marine amphibious force to respond to emergencies in vital areas like the Persian Gulf. Brown said that specialization of such a military force is limited by the variety of threats the United States must plan for all around the world.

"Our forces need to be jultipurpose," he said. "We're not like the Soviets, who have 150 divisions.They can deploy 35 or 40 on one border, 30 or 10 on another, and a big mass in between."

He said that the Second Army Division now being withdrawn from South Korea could be part of a specialized Persian Gulf force.

In contrast to the rise in danger of superpower conflicts within developing countries, Brown said that he does not believe the United States and the Soviet Union have gotten any closer to a nuclear war with each other in the last two decades. "I hesitate to say" that the risks of such nuclear conflict "have gone down. but they may have gone down," he said.

Brown said the most positive accomplishment of his two years at the Pentagon is the absence of U.S. involvement in any war.

"Now that's not something you can credit to the Defense Department," Brown stressed, "let alone to me. But it is a fact and it is something for which the administration as a whole intends to take full credit."

In the interview, the defense secretary made these observations on other major topics:

SALT II -- U.S. spending for startegic forces, those that would be used in an all-out nuclear war, will have to be increased above the present $10-billion-a-year level even if a new strategic arms limitation treaty is signed with the Soviets. With a new agreement, Brown said, the strategic budget would hav eto be increased 20 to 40 percent. Without the agreement, the increase would have to be between 50 and 60 percent, he said.

China -- "The big payoff militarily [in establishing relations with Peking] is that it stabilizes the situation in a way that reduces our concern about northeast Asia. But I think it would be a mistake to say the main payoff is military. Iths principally diplomatic." He added, however, that "it's very important to us that the Soviets not be able to throw their poolitical and military strength all at one point," which could be the case if the Soviets reached such an accommodation with China that they could transfer divisions on that border to the NATO front.

Space Warfare -- The Soviets have developed and tested anti-satellite weapons "and we haven't responded in kind. But that has not caused them to restrain" their antisatellite program. "So we are going ahead, still on a rather modest scale, but we've got some pretty good technology that would probably enable us to come out ahead."

Civil Defense -- "I don't think civil defense, whether it be by shelter in the cities or by evacuation" from them, "really prevents unacceptable damage from being inflicted on either side" in a nuclear war. "On the other hand, an outcome that differs by 30 or 40 million more dead in one case than in another in a given country, which could be the difference between the civil defense plan and no civil defense plan, could -- I don't think it's likely but it's possible -- affect perceptions and actions on the two sides. And so I think iths worth exploring" under a modestly funded program.

MZ Land Missile -- "Something has to be done" to make U.S. land-based missiles less vulnerable to Soviet attack. Whether to deploy the new MX land missile in aircraft or inside vertical shelters is still being studied. "I think that the situation in which our land-based missile systems can all be knocked out by theirs, and theirs cannot be knocked out by ours, would be a situation that would have very negative effects on perceptions of our capabilities...."