The Food and Drug Administration has decided to halt the marketing of fish oil capsules, concluding that they offer no known medical benefits.

"At the present time, there is inadequate scientific evidence to support health claims on fish oils or to support claims that these ingredients have an effect on the risk of coronary heart disease," the agency told manufacturers. It added that the safety of the product when taken over a long period had not been proved.

In a letter sent to 67 fish oil suppliers June 28 and released to the press yesterday, the agency ordered suppliers to cease distribution of fish oil supplements or face legal action.

Current inventories of fish oil capsules already on store shelves do not have to be withdrawn, agency spokesman William Grigg said.

Studies released several years ago found that Eskimos and other societies that consume large amounts of oily fish, such as in Japan and Greenland, have low serum cholesterol levels and few heart attacks.

In 1988 two trade associations of vitamin manufacturers applied to the FDA for permission to state on product labels that fish oil, which contains omega-3 fatty acids, has been shown to reduce the risk of heart disease.

But a subsequent study at the Minneapolis Veterans Affairs Medical Center found that men with high cholesterol counts who took high daily doses of fish oil actually increased their cholesterol counts.

In a June 20 letter to the two associations, the FDA said that the application had taken "a number of scientific conclusions out of context" to support its claims. "FDA has not listed omega-3 polyunsaturated acids as approved food additives or as being generally recognized as safe."