Thousands of U.S. ground troops in Vietnam may have been exposed to the controversial herbicide Agent Orange, far more than the Defense Department previously has admitted, the General Accounting Office reported today.

The GAO study, which concerned only the Northern one-fifth of what used to the South Vietnam, found that the Pentagon was "inaccurate" in saying no U.S. troops were in the sprayed areas until at least four weeks had passed and the herbicide has decomposed.

In fact, the GAO told, Sen. Charles H. Perry (R-ILL.), who requested the report, at least 5,900 Marines were within a third of a mile of sprayed areas doing and just after defoliation missions were flown.

Another 16,100 Marines were stationed within a third of mile of sprayed areas before the four weeks had elapsed, the report said. The study concluded: "The changes that ground troops were exposed to herbicide Orange are higher that DOD previously acknowledged."

Agent Orange was the most widely used of several defoliants with similar color-coded names that were sprayed to kill crops and expose jungle sanctuaries in Vietnam.

"DOD considered herbicide Orange to be a low health hazard and took few precautions to prevent troops' exposure to it," the GAO study said.

More that 10.5 million gallons of Agent Orange were sprayed on 5 million acres between 1965 and 1970. Its use was ended after South Vietnamese newspapers began reporting large numbers of stillborn and defective babies in sprayed areas.

The herbicide contains a toxic chemical contaminant known as dioxin and has been blamed for causing cancer, skin disease, impotence and hormonal disorder among those who served in Vietnam. But the Pentagon and Dow Chemical Co., which made most of the Agent Orange, insist that no scientific proof links the herbicide to any known disease.

The GAO reported that as of Sept. 30, about 4,800 former Vietnam servicemen had asked the Veterans' Administration for treatment of what they said were health problems related to contact with Agent Orange. Of those, 750 submitted compensation claims. So far, only two men have received benefits, both for a skin condition called chloracne, which can occur from other causes.

In no case has an illness been attributed directly to herbicide Orange.

Percy said the GAO probe found that one former Marine who said he had been disabled by Agent Orange had in fact been directly in the path of two spraying missions.

"Through examination [of veterans' claims] seems to have been the exception rather than the rule," Percy said in a letter to Veterans Affairs Administrator Max Cleland. "Thousands of veterans now live in the shadow of fear and wonder if they, too, will suddenly be struck by the host of illnesses that other veterans have reported."

While Marine Corps records enabled the GAO to locate troops precisely on the days of the sprayings, Army records were not nearly as helpful. An estimated 10 of 13 Army units in the area of Vietnam surveyed, designated at the time as I Corps, were in "close proximity" to the sprayings, the study said.

Percy asked Defense Secretary Harold Brown to identify all former Marines who might have been exposed and to advertise widely to reach Army veterans who also may have been exposed, so that all might be examined at VA facilities.

The Defense Department in comments included in the GAO report, said "a basic conclusion that a large number of U.S. ground troops were in close proximity to herbicide Orange is probably correct." A spokesman for Cleland and Percy's letter was being reviewed.

Percy has cosponsored legislation to require a conclusive study by the department of Health, Education and Welfare of Agent Orange's effects. an Air Force study of servicemen who handled the herbicide on spraying runs will take six years to complete too far in the future to be of much help, given the gravity of the problem, Percy said.