The United States has "a tacit obligation to back up" Iranian forces if the Soviet Union should invade that country.Air Force Secretary John C. Stetson said yesterday.

He added that the "collective wisdom" in the United States "would suggest that we would come to their defense," Stetson said, calling the U.S. obligation to Iran "a good deal more than moral."

"Whether it's an absolute commitment," advised the secretary, who visited Iran and other countries in the area last week, is a question for the international lawyers to settle.

A State Department official said calling the U.S. obligation to defend Iran "tacit" represented an unusually strong expression for an administration official to use.

After Stetson's press conference, Pantagon officials sought to soften the impact of the secretary's remarks about going to the aid of Iran by stressing he was giving his personal views. These Pentagon officials added that the United States has no treaty that "automatically" requires it to defend Iran.

A 1958 agreement of cooperation includes the United States and Ian as signatories, but Congress would have to approve before any forces could be committed to Iranian defense.

Asked who has there was a growing [WORDS ILLEGIBLE] to Iran from the Soviet Union, Stetson said. "The threat is reduced to the extent" that the Soviets perceive any attack would be unwise. He said a vacuum of power in the Persian Gulf area would be an invitation for the Soviets.

In that context, Stetson said Saudi Arabia not only needs the 60 F-15 fighter planes President Carter wants to supply but "quite likely" will need more aircraft. The Air Force secretary termed the 60 F15s a "drop in the bucket" considering the size of the country the Saudis have to defend and the value of their resources.

The Carter administration is preparing a military force that could go to war quickly in the Persian Gulf. Stetson said the Air Force will contribute to that force. Planning is in the preliminary stage, but the force is expected to include one Marine and two Army divisions with a total of about 100,000 troops.