The Department of Justice, rebuffed again by lower courts, will ask the Supreme Court Saturday for an emergency delay in the trial of former White House aide Oliver L. North because officials feel the case "now poses grave threats to national security."

Turned down a second time Friday by the U.S. Circuit Court of Appeals here, the department was preparing to appeal to the justices Saturday morning, hoping for swift action that could prevent the disclosure of sensitive government secrets at the Iran-Contra trial Monday.The trial is scheduled to resume at 9:30 a.m. Monday, and Department of Justice officials have said they fear North's lawyers will begin to disclose government secrets - to help bolster his legal defense - before the day's court session is over.

The appeal, Attorney General Richard L. Thornburgh indicated in a statement, is not designed to prevent a trial of North on charges growing out of the Iran-Contra scandal. The sole aim, he said, is to see that the trial follows exactly the provisions of a 1980 federal law that controls the handling of classified information in criminal trials.

"Unfortunately," Thornburgh said, "the judge in this case (U.S. District Judge Gerhard A. Gesell) has chosen not to follow the provisions of that law. Instead, he has adopted procedures of his own which do not provide the needed assurances of protection to sensitive matters."

The department wants a delay of the trial until its lawyers have a chance to ask higher courts to impose strict new procedures to prevent any disclosures of government secrets by North's defense lawyers.

Saturday's appeal is not the last maneuver available to the Department of Justice to get the assurances that Thornburgh and government intelligence agencies are demanding.

But the options that would be left if the Supreme Court also refused to delay the trial would threaten to wipe out some, if not all, of the 12 criminal charges remaining against North. Already, concern over the potential release of secrets has led to the dismissal of the two main charges against the retired Marine officer.

The first option - one that the Circuit Court of Appeals itself mentioned Friday - is a formal and direct ban by Thornburgh on "any classified information" that he wants to protect.

But the Department of Justice insisted Friday, in court papers, that it did not have specific information about what secrets might be at risk, so it could not draft such a ban.

If it issued a ban on "general categories," the department said, it might "imperil the prosecution." The reason is that a broad ban could lead Gesell to throw out some or all of the charges rather than deny North a right to use whole categories of such information to defend himself.

Another option would be for Thornburgh to force, directly, the dismissal of specific charges that seem likely to be keyed to classified information that North's lawyers might want to use.

The department, while pursuing appeals in hopes of getting a pretrial arrangement to protect secrets at the trial, has said that it is keeping open all of its options.