Declaring the heart of the government's case against Oliver L. North "destroyed," attorneys for the fired White House aide are stepping up attacks on the other 12 criminal charges he faces in the Iran-Contra affair.

Independent counsel Lawrence Walsh on Thursday moved to drop conspiracy and theft counts against North, citing the ex-Marine's "insistence on introducing classified information in his defense" and the Reagan administration's refusal to release classified material a judge said North needs to get a fair trial.President Reagan said Walsh's motion "satisfies our problem, which has been . . . concern about national security. We're waiting to see what the future is going to be."

Abandoning the two central charges while keeping the other 12 will "minimize the exposure of classified information," Walsh said.

North's attorneys disagreed with Walsh.

"Although the heart of its case is destroyed, the independent counsel continues to toy with Colonel North and apparently refuses to recognize that classified information pervades the remaining charges as well," Brendan Sullivan, one of North's lawyers, said in a statement.

Sullivan said "it is disgraceful" that the conspiracy and theft charges were brought against North "and outrageous that they survived for nine months after indictment." He said the proof of North's innocence lies in the classified material itself.

Walsh's motion was triggered by U.S. District Judge Gerhard Gesell's refusal to allow more censoring of 300 prosecution exhibits to meet administration concerns over disclosing sensitive information about covert intelligence operations.

Walsh says North, a former National Security Council aide, should stand trial on the 12 other charges dealing mainly with the alleged cover-up by North of his efforts on behalf of the Contras fighting the leftist government of Nicaragua. The 12 counts carry total penalties of 60 years in prison and $3 million in fines.

At a hearing Monday, Gesell is to consider the prosecution's request to dismiss the conspiracy and theft charges, which accuse North and three co-defendants of illegally diverting more than $14 million in U.S.-Iran arms-sale profits to the Nicaraguan rebels.

Walsh wants the two charges dismissed without prejudice, an approach that would make it possible to bring them again.

Gesell also will hear comments from prosecutors and North's lawyers on possible scheduling changes. Dropping the two main counts will require significant adjustments in the documentary case both sides plan to present. The trial is set to begin Jan. 31.

House Majority Leader Thomas Foley said the decision to drop the two charges "has nothing to do with the inadequacy of the evidence. It has to do with the sensitivity of the evidence."

"Col. North still faces very serious charges," said Foley, D-Wash. "The dismissal is not any sort of vindication."

Reagan was asked whether he was going to pardon North and responded, "We're waiting for the judicial process to go forward."

Arthur Kropp, president of the liberal group People for the American Way, said "the Reagan administration appears determined to give Ollie North a `pocket'pardon as a reward for his Iran-Contra activities."

Attorney General Dick Thornburgh denied such contentions, saying "the assurance that was given to me by Judge Walsh was that he had been dealt with in good faith, that every line of inquiry that he pursued he received the fullest cooperation from the executive branch."