Possible disclosure of some of the nation's most closely guarded secrets could prevent prosecution of Oliver North and three others charged in the Iran-Contra scandal, a judge warns.

At a pretrial hearing Monday, U.S. District Court Judge Gerhard Gesell said a federal law restricting the use of classified information poses a major problem to prosecution of the Iran-Contra defendants.Under the the Classified Information Procedures Act, the defense must tell the prosecution what classified material it plans to use at trial and is barred from using the information if it fails to give advance notice.

Defense lawyers argued the federal provision essentially forces them to disclose the heart of their case before trial and urged Gesell to strike down the law.

But independent prosecutor Lawrence Walsh said there are only about a dozen categories of classified information that could cause problems at trial and suggested ways to overcome those obstacles.

Gesell, who said he would rule on the issue at a later date, questioned how "one could have a trial where everyone writes out what they're going to do" in advance.

"Do we have a try-able indictment?" Gesell asked.

North and his codefendants - former national security adviser John Poindexter, Iranian-born businessman Albert Hakim and retired Air Force Maj. Gen. Richard Secord - are charged with conspiring to defraud the government by diverting profits to the Nicaraguan rebels from the sale of U.S. arms to Iran.

The judge, who presided over much of the Watergate legal action 14 years ago, reiterated his concern about whether the prosecution would be able to pursue the central charge in its 23-count indictment, conspiracy.

Gesell said the conspiracy counts are "related to a wide course of conduct" that could prompt defendants to draw comparisons with other covert operations in their testimony. But he questioned to what extent a defendant would be able to testify on his own behalf given the restrictions on disclosure of classified material.