President Reagan's national security affairs adviser, Robert C. McFarlane, told a private meeting of businessmen last week he believes that some servicemen missing in action in Vietnam are still alive and said the United States does not have enough "human intelligence" in Vietnam.

McFarlane, whose comments were reported this week by The Wall Street Journal, was offering "a personal viewpoint and a personal judgment," White House spokesman Larry Speakes said yesterday.

"Our position is there has been no concrete evidence of living Americans from the Vietnam war being held in Vietnam but on the other hand we have had an active effort on the part of the government" to follow up on leads about missing servicemen, Speakes said.

McFarlane was speaking last Wednesday to an off-the-record forum for business executives sponsored by syndicated columnists Rowland Evans and Robert Novak, the Journal said. His remarks were tape recorded, however, by former House member John LeBoutillier (R-N.Y.), and made available to the Journal.

McFarlane said he was giving his personal view -- "how I really feel about it. I think there have to be live Americans there." He said there was no conclusive proof, but "there is quite a lot of evidence given by people who have no ulterior motives and no reason to lie, and they're telling things that they have seen."

McFarlane was critical of U.S. efforts to find out what happened to nearly 2,500 Americans still unaccounted for, the Journal reported. "What we need to do is have better human intelligence in Vietnam. Now we don't. It takes time to get it. But I wouldn't pretend to you that we have done enough to even start. And that's bad. That's a failure."

Pentagon spokesman Robert B. Sims said this week that locating Americans still listed as unaccounted for remains a priority of the administration.

McFarlane told the businessmen he believed that the administration has made progress by continuing to press Vietnam for an accounting of the missing servicemen, the Journal said, and he indicated that U.S. efforts to collect better intelligence would be stepped up.

"We are doing more," he said, " but there is more that we ought to be doing than we are. And I think we shall very soon."