MOSCOW, April 11— A document described as a top secret report written by a senior North
Vietnamese general and delivered to the Communist Party Politburo in Hanoi in September 1972 says that North Vietnam was holding 1,205 American prisoners of war when Hanoi maintained that the number was only 368.
A copy of the report, a Russian translation of the Vietnamese document, was recently discovered here in the archives of the Soviet Communist Party.
The report has been circulated among United States Government officials. It has been described as authentic by experts, some of whom have called it a "smoking gun" that proves Hanoi has been withholding information about the fate of American prisoners of war in Vietnam.
Discovered in January by a Harvard University researcher, the document gives a detailed accounting of 1,205 prisoners held in 11 North Vietnamese prisons in the fall of 1972, at a time when the peace talks were under way in Paris.
The author of the report, General Tran Van Quang, then Deputy Chief of Staff of the North Vietnamese Army, said in the document: "1,205 American prisoners of war located in the prisons of North Vietnam -- this is a big number. Officially, until now, we published a list of only 368 prisoners of war, the rest we have not revealed. The Government of the U.S.A. knows this well, but it does not know the exact number of prisoners of war, and can only make guesses based on its losses. That is why we are keeping the number of prisoners of war secret, in accordance with the Politburo's instructions."
Several months later, under the peace agreement between North Vietnam and the United States, 591 P.O.W.'s were released. When the last of those prisoners were freed on April 1, 1973, Hanoi said that no more American prisoners were left in Vietnam -- an assertion that it has maintained.
But the Sept. 12, 1972, report suggests that North Vietnam was either withholding prisoners after the peace agreement was reached or, if the prisoners were no longer alive, knew about their fate. More Than 700 Held Back
"On the basis of this, we can conclude that more than 700 Americans had been held back by the Vietnamese at the time of Operation Homecoming," said Stephen J. Morris, a 44-year-old researcher for the Harvard Center for International Affairs and the Russian Research Center at Harvard, who discovered the document.
"This is the biggest hostage-taking in the history of American foreign policy and we still don't know where the hostages are, what happened to them, if they are still alive," said Mr. Morris, who is working on his second book about the Vietnam War.
Members of a American-Russian commission investigating the fate of other American P.O.W.'s, captured by either the Soviet Union or its allies, say the document is authentic. The Russian newspaper Izvestia reported Saturday that the document on the American prisoners in Vietnam, along with other newly declassified material, was discussed at a recent closed meeting of the commission.
The document could complicate Hanoi's efforts to establish diplomatic relations with the United States and have Washington end its 18-year trade embargo. The White House announced Saturday that John W. Vessey Jr. a retired general, would visit Vietnam on April 18 and 19 to assess Hanoi's cooperation in accounting for missing servicemen. Marked 'Top Secret' in Russian
The document, found in the Communist Party archives, consists of both a Russian translation of General Quang's full report and a summary prepared by the Soviet Army Intelligence Agency. It is marked "Top Secret" in Russian, and on the first page of the summary it shows handwritten instructions for a "brief note . . . on the prisoners of war" to be dispatched to the Soviet Politburo.
Experts on the issue, which has haunted relatives of servicemen listed as missing in action and a succession of Washington Administrations, say one element strongly suggesting that the document is authentic is the reference to the figure of 368, which was the number of P.O.W.'s given to American representatives at the Paris peace talks. Furthermore, one Congressional expert, who spoke on the condition of anonymity, said the number of 1,205 more or less corresponds to the number of prisoners the United States was expecting to see returned.
The United States still lists more than 2,200 servicemen from the Vietnam War as unaccounted for, although more than half were known to have died. The Government says the fate of the others is not known.
A Senate committee that investigated the issue reported in January that "there is no proof that United States P.O.W.'s survived, but neither is there proof that all of those who did not return had died."
Mr. Morris said he first showed the report to top White House officials in February. According to a person who works for a Senate committee, a report on the Soviet document was delivered to President Clinton on the eve of his meeting with President Boris N. Yeltsin of Russia in Vancouver on April 3 and 4. It is not known whether the subject was raised at the meeting. No White House Evaluation Yet
A Clinton Administration official said tonight of the report of the document, "We are pursuing it very seriously but are not in a position to evaluate it."
He also said that the American researcher had informed the Government of his discovery but that "he would not give us the document."
He added that the former Ambassador to Moscow, Malcolm Toon, had been asked to follow up the matter in Moscow. As to its impact on United States-Vietnamese relations, the official said, "I assume it is something Vessey will raise."
Alan C. Ptak, former Department of Defense Deputy Assistant Secretary for P.O.W./M.I.A. Affairs, said tonight that he had not heard of the document but that he did not find it surprising. "I always had an inkling that we hadn't been told everything," he said. "We had suspicions the Russians knew more than they were telling us."
In his report, General Quang said the prisoners could be freed only as a part of an overall peace settlement and could be used as leverage to obtain compensation for the devastation caused by the war.