The White House said yesterday that the United States has not altered its long-standing plan to deploy a new generation of significantly more powerful and accurate nuclear warheads atop its fleet of Minuteman III intercontinental ballistic missiles.

"We do not feel it is reasonable to expect this county unilaterally to forgo all technological (weapons) improvements . . . absent similar restraint by the Soviet Union," White House press secretary Jody Powell said of the planned deployment.

Powell, answering reporters' questions, was responding to an article in The New York Times which he later said had unfairly suggested that the United States was unilaterally escalating the arms race.

But Powell, in the process of attempting to correct what he said was an erroneous impression left by the newspaper article, managed to produce some confusion on his own.

For example, he denied that development of the new warhead is a "bargaining chip" in strategic arms limitation talks with the Soviets, but suggested that the United States might reconsider the deployment if the right kind of response from the Soviets is forthcoming.

"There is a possibility that if the Soviets want to negotiate seriously we can make changes in our plan," Powell said.

He also said that President Carter's decision on deployment of the warhead had been "fairly recent." In fact, Powell later said in correcting himself, the decision, such as it was, occurred last February. It amounted to a decision not to do something - not to remove funds in the budget for continuation of the deployment process.

At issue during the questioning in the White House briefing room was the Mark-12A nuclear warhead. Under development for the last five years, the weapon carries roughly twice the explosive power of the current warheads atop Minuteman III missiles and has a much more sophisticated guidance system, greatly improving its accuracy.

The result of this greater power and accuracy would be to increase significantly the ability of American missiles to destroy land-based Soviet missiles in their hardened, protective underground silos.

Powell conceded that the Mark-12A would give the United States a "temporary advantage" over the Soviet Union in strategic weapons, but he denied that its deployment would amount to a unilateral escalation of the arms race. Absent mutual restraint by both sides, he said, if one or the other is to enjoy a temporary advantage, better it be the United States than the Soviet Union.

Powell said that the comprehensive arms limitation proposal that the United States had made to the Soviet Union - and which the Soviets initially rejected - contained a section on qualitative improvements to weapons systems designed to deal with issues such as the Mark-12A. There have been no serious negotiations on that specific issue although there are "serious discussions in progress" on arms limitations in general, he said."

"The longer we wait to move forward [on arms limitions] the more we and the Soviets and the world are going to be faced with situations like this," Powell said.

Dr. Jeremy J. Stone, director of the Federation of American Scientists, has long argued that deployment of the Mark-12A will give the United States a "first strike" capability in its nuclear forces and greatly expand the arms race as the Soviet Union seeks to match it.