A judge Friday rejected Oliver North's remaining challenges to Iran-Contra charges against him, but the Reagan administration's refusal to release classified documents could still scuttle the major charges.

U.S. District Judge Gerhard A. Gesell rejected the last of the former White House aide's pre-trial motions. North had moved to dismiss charges that he obstructed congressional inquiries into press reports he was violating a ban on helping the Nicaraguan rebels, known as Contras.Gesell's rulings capped a series of decisions on North's 37 pre-trial motions that left intact 14 of the 16 charges against the former National Security Council aide.

But major counts against North could be dropped if Gesell decides the defense cannot argue its case without classified documents being withheld by the Reagan administration.

President Reagan insists that the documents are being withheld for national security reasons and says the decision is not a ploy to prevent North from going to trial.

"The things we're blocking are the things that duty requires we block," Reagan said Thursday. "These are things that are national security secrets."

Asked if the move were a backdoor maneuver to keep the case against North from going to trial, he replied: "No, this is something that from the very beginning we knew we would have to do."

Reagan was questioned at the beginning of a meeting on forthcoming trade talks. Although he defended the decision to withhold documents from the trial, he backed away from his previous insistence that North and his former boss, John M. Poindexter, are innocent.

"At this stage I don't think I should comment on guilt or innocence or anything of this kind. I think the law has got to take its course," Reagan said Thursday.

Still, the decision to withhold classified documents could enable North's lawyers to claim that he is being prevented from presenting a full defense to the key allegation against him - that he conspired illegally to divert more than $14 million in profits from the Reagan administration's secret Iranian weapons deals to the Nicaraguan Contra rebels.

North's lawyers claim that they need some 40,000 pages of secret documents to defend him. Independent counsel Lawrence Walsh, the special prosecutor assigned to the case, has indicated he can proceed without use of the classified information.

White House spokesman Marlin Fitzwater said heads of government agencies concerned with national security decided in July not to release some material sought for the trial. He said Walsh indicated at the time that he could prosecute North on the basis of unclassified material.

North's lawyers, however, are insisting on use of the classified material. Gesell resumed closed hearings Friday on defense objections to deleting classified material from 350 prosecution exhibits.

The secret materials bear most heavily on two charges relating to the diversion - conspiracy and theft of government property. However, North also could be tried on charges he made false statements to Congress and conspired to use a tax-exempt foundation for a non-exempt purpose - raising money for the Contras.

In a court filing made public late Thursday, Walsh disclosed some of the types of information that the Reagan administration is preventing him from using. It includes the already widely publicized names of several Central American countries, and the names of officials from those nations, involved in aiding the Contras.

Walsh's document also indicates that the administration is trying to suppress the names of nations from which it tried to solicit secret contributions for the Contras, including Saudi Arabia and South Korea.

Fitzwater said the White House has offered a briefing to Gesell but had not received a response. Fitzwater said it was "up to the judge" to decide if the classified material is relevant to North's defense and, if it is, whether the indictments against him should be dismissed or he should be tried.> Reagan, meanwhile, reiterated that he is opposed to a presidential pardon for North and the other Iran-Contra defendants while the criminal case is pending. He said pardons would leave them "under a shadow of guilt for the rest of their lives."