An 11th-hour attempt by the National Security Agency to block the Iran-Contra trial of former White House aide Oliver L. North on grounds that it could reveal damaging national secrets was rejected last week by Attorney General Dick Thornburgh, the Los Angeles Times learned Saturday.

Government sources said the appeal by the super-secret security agency failed to gain the support of its fellow members on an interagency intelligence panel. That panel's earlier objections had led to the dropping of two main charges against North two weeks ago.But the National Security Agency decided to press the matter with Thornburgh, according to an administration source. Under the Classified Information Procedures Act, the attorney general is the top government official with authority to prevent disclosure of sensitive information at the trial, which is set to open Tuesday.

The specific objections raised by the agency, which conducts worldwide electronic eavesdropping and code-breaking activities from its Fort Meade, Md., headquarters, could not be learned. But government sources said they involved matters raised by some of the 300 documents that his lawyers plan to use to defend North, a former National Security Council aide.

The interagency intelligence panel has been reviewing defense material and the avenues of testimony the documents are expected to open. In addition to the National Security Agency, the panel includes representatives of the CIA and the departments of State, Defense and Justice.

An official at the National Security Agency's operations center Saturday declined to discuss the matter.

David Runkel, Thornburgh's chief spokesman, said "there is nothing pending at the present time" with the attorney general that could delay the trial. "As far as we're concerned there's no inhibition" to the trial opening on schedule, he said. Runkel would not confirm or deny that the National Security Agency had appealed to Thornburgh.

James Wieghart, a spokesman for independent counsel Lawrence E. Walsh, said, "We are prepared to go to trial and expect to Tuesday." He said he could not discuss matters involving the interagency committee.

The interagency panel's review of North's defense documents appears to be the last substantial hurdle to the trial getting under way, and the National Security Agency's decision to lodge its own appeal with Thornburgh signaled that the panel will raise no further objections before the proceedings begin.

North's attorneys are seeking to subpoena former President Reagan and President Bush to testify at the trial. But U.S. District Judge Gerhard A. Gesell on Friday postponed a decision on whether to uphold the defense subpoenas for their testimony and relevant documents, and the question is not expected to delay the trial opening.

North's lawyers would like to prove that North took part in several covert operations similar to the Iran arms initiative. They hope that jurors might conclude that North believed that selling arms to Iran to win release of U.S. hostages was entirely legal, since the other projects had been authorized.

It was not clear that the National Security Agency objections involved covert operations that North took part in as a National Security Council aide. But the agency traditionally has been the most sensitive on the interagency panel about disclosing intelligence sources and methods.

Acting on objections raised by the interagency panel to classified information that Walsh's prosecutors sought to disclose at the trial, Gesell on Jan. 13 dismissed the two main charges of conspiracy and theft against North after Thornburgh certified that government secrets would be compromised.