WASHINGTON (AP) _ The Republican leader of the Senate panel investigating Americans missing from the Vietnam War charged Tuesday that federal agencies have been withholding documents from the committee.

Sen. Bob Smith, R-N.H., said he wants to subpoena the documents before the committee goes out of existence Jan. 3.

''We have hundreds of questions yet to be answered, thousands of documents that this committee has not seen and that this government refuses to provide,'' he said at the opening of four days of hearings.

Sen. John Kerry, D-Mass., chairman of the Senate Select Committee on POW- MIA Affairs, said that while he has been at times ''somewhat distressed'' with the Pentagon's level of cooperation, he was satisfied with its overall efforts.

''Any document we ought to have, we get,'' he said. ''The (committee's) effort thus far has been extensive and extraordinary and the response has been extensive.''

The documents Smith wants include 60 boxes of intelligence reports from the National Security Agency, operational files from the Central Intelligence Agency, debriefing reports from returned prisoners of war and more than 500 Defense Intelligence Agency reports claiming POWs have been alive in captivity in Vietnam and Laos as late as 1989.

Sen. Charles Grassley, R-Iowa, said that while the committee has worked for nearly a year trying to find out what happened to U.S. servicemen who didn't return from Vietnam, ''our attempts have been frustrated by prejudgments.''

He blamed the Pentagon, saying, ''the mindset to debunk (reports of sightings of POWs and MIAs) is alive and well at the Department of Defense.''

On the first of four days of hearings this week, the committee was told that the Defense Intelligence Agency's office on POWs and MIAs was hampered by management problems, lack of resources, lack of leadership and pressures from politicians and activists.

The authors of several Pentagon reports critical of the DIA in the 1980s testified that these failings combined with fabricated information caused investigators to become overly cynical.

''A degree of cynicism crept into our analysis,'' said retired Rear Adm. Thomas Brooks, who wrote a memorandum sharply critical of DIA operations in September 1985 after serving as a flag-rank officer in charge of the agency's POW-MIA office for several months earlier that year.

Though he said the cynicism was ''part of human nature,'' he said ''it is not appropriate to intelligence efforts.''

Col. Kimball Gaines, a former supervisor of the POW-MIA office who conducted a review highly critical of the office in March 1986, said a major problem at the time was that DIA analysts had no buffer between themselves and members of Congress and outside interests, including activists.

''They were constantly running from one crisis to another and didn't have time to do the digging,'' he said.

Retired Lt. Gen. Leonard Perroots, who headed the DIA from 1985 to 1988, said part of the problem was that the agency was inadequately financed, but he said another problem was that too many materials were classified. ''The consequence was an impression to the public that DOD (Department of Defense) or the government had something to hide.''

He and all of the other current and former Pentagon officials who testified said they had never seen any evidence to suggest that the government was covering up information on POWs or MIAs.