U.S. officials yesterday reacted with bafflement and curiosity to Russian President Boris Yeltsin's disclosure that U.S. servicemen captured in the Vietnam War may have been secretly held in the Soviet Union.

The possibility that U.S. soldiers were taken to the Soviet Union -- and may even still be alive -- "does not square with what we thought we knew," said retired Army Gen. John W. Vessey Jr., special presidential envoy to Vietnam for POW/MIAs. "It's absolutely new information."

Yeltsin made the disclosure in an interview with NBC News Monday and apparently reiterated it yesterday in a meeting with President Bush at the White House. Yeltsin has yet to offer any evidence for the claim, and following Monday's news report, the senior Russian member of a joint U.S.-Soviet commission on POW-MIA issues expressed some surprise at the Russian president's remarks.

Nevertheless, while administration officials were skeptical, Bush yesterday promised to dispatch "immediately" to Moscow members of the U.S.-Russian commission to follow up Yeltsin's tip.

Yeltsin's comments to Bush followed a remarkable series of disclosures from the Russian president, not just about possible Soviet complicity in the disappearance of U.S. servicemen in Vietnam, but throughout the Cold War. Last week, Yeltsin said in a letter to the Senate Select Committee on POW-MIA Affairs that the Soviets had kept 12 Americans shot down on spy missions over Soviet territory in the 1950s in prisons and psychiatric hospitals.

Pete Williams, the Pentagon's chief spokesman, yesterday acknowledged that some aircraft were lost over Soviet territory during that period in previously undisclosed incidents. He said, however, that the U.S. government is still not clear about which downings the Russian president was referring to in his letter and does not have the names of any of those held.

"We are asking now for more information from the Russian side, so that we can try to correlate whatever information they may have with information that we have," Williams said.

Of particular interest to the Pentagon, Williams said, was another report reaching the U.S. government several days ago through undisclosed channels that an American serviceman from the Korean War may still be alive in Russia. Williams said the report was being investigated but declined to name the soldier or provide any details about his purported whereabouts.

Administration officials have for the most part discounted persistent rumors that U.S. soldiers may have been held in Vietnam or the Soviet Union for years after the end of the war. But the issue has been kept alive by some POW-MIA advocates, several of whom have already seized on Yeltsin's disclosure as a vindication of their long-held belief that the U.S. government ignored or suppressed evidence that American prisoners may still be alive.

"This is what we've been fighting for," said Dolores Apodaca-Alfond, chairman of the National Alliance of Families and sister of an Air Force major who disappeared along with his F-4 fighter June 8, 1967. "Now we want to see the whites of their eyes. We want to see American POWs on American soil."

A more cautious note was sounded by Ann Mills Griffiths, executive director of the National League of Families of American Prisoners and Missing in Southeast Asia. "My concern is this could easily mislead people," she said. "We all are waiting anxiously to hear clarification and determine what this information really pertains to."

Yeltsin first acknowledged the possibility of a Soviet role in the POW-MIA mystery in an interview with NBC News on his flight here Monday. But because the information had not been included in his letter last week to the Senate committee, many U.S. officials were inclined to dismiss his comments as the result of a misunderstood question or translation problem.

But Yeltsin apparently reiterated his statement on the Vietnam POWs during his meeting with Bush at the White House yesterday. "President Yeltsin informed me for the first time that Russia may have information about the fate of some of our servicemen from Vietnam," Bush said at a news conference.

Bush declined to offer any details of his conversation with Yeltsin on the missing soldiers, saying only, "He has told me he will go the last mile to find whatever it is that exists about our possible . . . American POWs and MIAs."

Bush said he had asked the U.S. co-chairman of the Russian-U.S. commission on POW-MIAs, Malcolm Toon, to "return immediately to Moscow to work on this issue."

The confusion over the fate of the American POWs in Russia reflected the protracted and tortuous process of declassifying hundreds of millions of formerly top-secret documents in archives belonging to the Soviet Communist Party and KGB security police. While Yeltsin administration officials have made a number of startling allegations, including a claim that the former Soviet regime had widespread contacts with terrorists, release of documents has been partial and very selective.

Establishing what happened to every person who passed through the system of Stalinist prison camps, known as the gulag, is virtually impossible, given the huge numbers involved and the lack of computer records. Russian presidential spokesman Vyacheslav Kostikov told reporters yesterday that a start had been made on identifying individual graves of U.S. servicemen.

U.S. GOVERNMENT ESTIMATES

War ............ Number MIAs

Vietnam .............. 2,266

Korea ................ 8,000

World War II ........ 78,000

SOURCE: News reports