MIA Disclosure a Political Shock : Vietnam War: Yeltsin’s assertions could create problems for Bush and complicate U.S. aid for Moscow.

Russian President Boris N. Yeltsin’s startling assertions about missing U.S. servicemen set off shock waves Tuesday that officials said could deepen public mistrust of government, create new diplomatic problems for the Bush Administration and breathe life into theories long dismissed as activists’ far-fetched conjecture.

Within hours of Yeltsin’s acknowledgment that Americans captured in the Vietnam War were held in the Soviet Union and that some may even be alive there today, President Bush vowed that a joint U.S.-Russian commission would waste no time in resolving their fates.

“If anyone’s alive, that person--those people--will be found,” the President said, adding that he is sending former U.S. Ambassador Malcolm Toon to Moscow immediately to investigate further.

But that promise was not enough to satisfy those on Capitol Hill who already were arguing that Russia should not receive any U.S. aid until it comes across with more than sketchy statements about the matter.

“If there are Americans being held in Russia, President Yeltsin should release them immediately,” said Sen. John McCain (R-Ariz.), a former Navy fighter pilot who was held prisoner in Hanoi. “If there are not, he should provide conclusive evidence proving this fact to the American people.”

Many activists said they see Yeltsin’s comments--first broadcast Monday night in an NBC News interview--as vindication of a cause that they believe their own government has failed to take seriously.

“I think that it’s sad that the revelation had to come from the Russian government and not our own government,” said Red McDaniel, head of a POW/MIA group known as the American Defense Institute. “Certainly, they’ve known. I think the (U.S.) government has been less than honest on this issue.”

Shelby Robertson Quast, the daughter of a missing pilot whose family claims to have recognized him in a photograph made public last year, observed: “I have a funny feeling that Yeltsin has no idea what kind of can of worms he opened. For the first time, our government might be forced to actively investigate this.”

With 2,266 U.S. servicemen still unaccounted for, almost two decades after the Vietnam War, POW / MIA organizations long have urged Washington to look to Moscow for information. They have argued that downed U.S. pilots, in particular, would have been of far more value to the Soviet Union than to Hanoi, because Moscow had more need for their knowledge of state-of-the-art military technology and plans.

Over the years, several former intelligence officials have come forward as well, contending that they had intercepted communications indicating that prisoners of war were regularly transferred to the Soviet Union. But the Pentagon generally has dismissed those reports, often attacking the credibility of the sources.

Mark Sauter, a journalist and author of a soon-to-be-published book on the subject, said Yeltsin’s acknowledgment that the Soviet Union held U.S. servicemen “creates a whole new agenda with this issue. It wipes away with one stroke 40 years of disinformation and inaction by the U.S. government.”

A senior White House official said the Administration is taking Yeltsin’s comments seriously but acknowledged that they have bewildered the White House. As recently as last week, Yeltsin had told the U.S. Senate that 12 American pilots were shot down by the Soviet Union and imprisoned in the 1950s, marking the first time that either country had acknowledged the incidents.

But he had made no mention of Vietnam-era servicemen. At first, surprised White House and congressional officials speculated that Yeltsin might have misspoken in his NBC interview, confusing Vietnam-era incidents with earlier ones. Yeltsin’s later statements, in public, and to Bush, left no doubt. The White House official speculated that Yeltsin’s comments were based on information he had obtained only in the last few days.

The U.S. official also expressed concern that the new information could make it more difficult to win approval of the Russian aid package that Bush has submitted to Congress. It also could complicate negotiations to restore diplomatic relations with Vietnam, which has insisted that it has returned all American prisoners to this country. He described this as a “very unfortunate linkage” and noted that Yeltsin has “promised to do everything he can” to resolve the matter.

In January, the United States and Russia established a joint commission to pore over information, long held secret by the fallen Soviet government, that could resolve the cases of servicemen listed as missing from World War II, the Korean War, the Vietnam War and the Cold War. A select Senate committee is also investigating the overall POW / MIA issue.

Sens. John Kerry (D-Mass.) and Robert C. Smith (R-N.H.), chairman and vice chairman of the POW / MIA panel, sent a letter to Yeltsin on Tuesday asking him to make time in his summit schedule to meet with their committee.

Vyacheslav V. Kostikov, Yeltsin’s press secretary, told a press briefing Tuesday evening that the new information about the American POWs was found only recently when formerly top-secret KGB and Communist Party archives were examined.

Although former Soviet President Mikhail S. Gorbachev proclaimed a policy of glasnost , or openness, those archives remained off-limits to the public during his Kremlin leadership, Kostikov noted.

“Only in the past months have those archives been opened,” Kostikov said.

The Russian spokesman said Bush was “touched and excited” at the information provided Tuesday by Yeltsin and sought to reciprocate by offering to help the Kremlin win the freedom of Soviet soldiers captured by insurgents during the Afghanistan war and held prisoner since.

Kostikov said there were “many different categories” among the 2,000-plus American soldiers that he said had been imprisoned at one time or another in the Soviet Union--some in the wake of World War II. For example, some World War II GIs with Russian names were wrongly believed to belong to the Vlasov Army created by the Nazis out of captured Red Army units, he said.

The graves of some American captives have already been found in Russia, Kostikov said.

Times staff writer John-Thor Dahlburg contributed to this report.