Newly declassified documents show that the United States knew immediately after the Korean War that North Korea had failed to turn over hundreds of American prisoners known to be alive at the end of the war, adding to growing speculation that American prisoners might still be alive and in custody there.

The documents, obtained from the Dwight D. Eisenhower Presidential Library and other Government depositories by a Congressional committee, show that the Pentagon knew in December 1953 that more than 900 American troops were alive at the end of the war but were never released by the North Koreans.

The documents may only deepen the mystery over the fate of Americans still considered missing from the Korean War. In June a Defense Department intelligence analyst testified that on the basis of ''a recent flurry'' of ''very compelling reports,'' he believed that as many as 15 Americans were still being held prisoner in North Korea.

While not dismissing the analyst's report entirely, the Defense Department has said it has no clear evidence that any Americans are being held against their will in North Korea, although it has pledged to continue to investigate accounts of defectors and others who say they have seen American prisoners there.