The Carter administration did not disclose Cuban Presidnet Fidel Castro's statement that he tried to head off the cross-border attack on Zaire's Shaba Province because policymakers did not believe Castro was telling the truth, a State Department official said yesterday.

Castro's statement on May 17 to Lyle F. Lane, the ranking U.S. diplomat in Havana, became known Friday after Lane's report on the meeting was made available by the State Department to the Senate Foreign Relations Committee.

Castro told the diplomat, according to informed sources, that he heard reports a month or more in advance that Lunda tribesmen baed in Angola were planning another foray into their homeland across the Zaire border. Castro said he had tried unsuccessfully to persuade the Angolan government of President Agostinho Neto, which is presumed to have influence over the Lundas (or Katangans, as they are often called) to put a stop to the assault.

President Carter on May 25 charged that Cuba shares with Angola a responsibility for the attack. "We believe that Cuba had known of the Katangan plan to invade and obviously did nothing to restrain them from crossing the border," said Carter in statement that he read to a news conference in Chicago.

A State Department official who was involved said the details of Castro's statements were known to those who drafted Carter's press conference remarks but that "it did not make a good impression on us." Calling the Castro claim "as weak as dishwater," the official said it was given no more credence than the Cuban leader denial of charges that Cubans had trained the cross-border invaders.

Several State Department officials said the full account of Castro's conversation with the U.S. diplomat was "very closely held" in the U.S. government. One high-level official involved in policy matters said he knew of Castro's admission of advance knowledge of the invasion, but had not been told that Castro asserted he had tried to stop it. A middle-rank official involved in African matters said the same thing.

An informed official said it had been a "rather routine" decision to supply the Senate Foreign Relations Committee with a copy of the classified report on Lane's May 17 meeting with Castro. The committee had requested access to the document through normal channels, and several senior State Department officials concurred in the decision to make it available, according to this source.