The Pentagon yesterday pulled back from the White House assertion on Wednesday that deploying the Mark 12A warhead atop the Minuteman III intercontinental ballistic missile would give the United States a "temporary advantage" over the Soviet Union in strategic weaponry.

Pentagon spokesman Thomas D. Ross said at the regular Thursday news briefing: 'Even if the Mark-12A were to work perfectly, we don't believe we would be able to knock out most of their [Soviet] silos because of our limited Minuteman force."

The Pentagon's "overall judgement," Ross continued is that "the Mark-12A would not be a threat to most of their land-based silos, while their force might well be a threat to our entire land-base force.

"In the absence of a balanced SALT [strategic] arms limitation talks agreement," Ross said, "we feel that we should go ahead with this program in order to maintain the current situation of rough equivalence."

In a related development, the Federation of American Scientists yesterday issued a statement signed by 11 arms specialists urging the Carter administration to defer continuing with the Minuteman III improvement program to avoid "foreclosing future agreement" at arms talks with the Soviets and to "presence our leverage" at those negotiations.