UNITED NATIONS, DEC. 8 -- The United Nations acknowledged today that more than 400 files on Nazi war crimes have been missing from its archives for at least a year, and said it has ordered an urgent investigation.

Disclosure that the U.N. archives staff knew of the missing files last year but did not tell the secretary general raised questions of whether a cover-up was involved. It followed a report today by the New York Post that 433 of the 8,100 recently opened files had disappeared.

Israel's U.N. ambassador said vital information on atrocities against Jews and others by the Nazis during World War II may have been lost or destroyed.

It was the second dramatic revelation about the archives of the U.N. War Crimes Commission, kept in a Park Avenue office building since World War II. In March 1986, it was discovered that the archive, with more than 36,000 names, contained a file on former U.N. secretary general Kurt Waldheim, now the president of Austria.

There also was talk of a cover-up then, a charge denied by the United Nations and archivists. Waldheim has denied any wrongdoing during his service in the German Army.

Elan Steinberg, director of the World Jewish Congress, said that with all the attention surrounding the file on Waldheim "it is bothersome that no one mentioned that these files were missing. They could be in a box somewhere. It is not necessarily sinister, except that they have known for a year. It was either a foul-up or a cover-up."

U.N. Secretary General Javier Perez de Cuellar issued a statement saying he was "surprised and disturbed" to learn for the first time, through news reports, that some files of the war crimes commission were missing. "This was not known in the United Nations Secretariat outside of the Archives Section," said the statement.

The U.N. chief ordered a full investigation by Richard Foran, assistant secretary general for general services. The inquiry will try to determine whether the missing files were separated from the archives while they were in U.N. custody, or whether they ever were received by the United Nations when it first got custody after the war.

It also will investigate whether the files concern adjourned and withdrawn cases, which are filed separately and are not on microfilm.

When asked whether there might have been a cover-up, U.N. spokesman Francois Giuliani said, "Of course not." He said it probably was last year during the microfilming of the archives that some files were found to be missing.

The Post said that some of the records, particularly those on the reel of microfilm containing Yugoslavia's Waldheim file, were missing in blocks of up to six consecutive documents.

It quoted Chief Archivist Alf Erlandsson as saying his department had known for more than a year about missing files on crimes and atrocities but told no one until questioned by the newspaper. He could not be contacted today.

Sources close to the archives said the files were missing from microfilm but said they might know where the material could be located.