Former national security adviser John M. Poindexter provided new details last week on President Reagan's knowledge of administration support for the Nicaraguan contras -- testimony that suggests Reagan's involvement in efforts to circumvent congressional restrictions on rebel aid was much deeper than previously known.

Poindexter testified that he briefed the president on a clandestine Costa Rican airstrip built by Lt. Col. Oliver L. North's covert contra support network, that he gave Reagan periodic reports on the rebels' status "in the field," and that the president knew that North was the "chief staff officer . . . carrying out his charter to keep the contras alive" during the two-year ban on most U.S. military aid.

The restrictions on aid, known as the Boland Amendment, were in effect between October 1984 and October 1986.

Poindexter supported the president's position that he was unaware that funds from the secret U.S.-Iran arms sales had been diverted to the contras.

But while the ban was in effect, Poindexter said, Reagan told him, " 'I don't want to pull out our support for the contras for any reason . . . . Isn't there something that I could do unilaterally?' "

Poindexter's statements to the congressional Iran-contra panels, combined with the testimony of North and others, have shown a president who at least had a deeper grasp of disparate details about North's contra support operation than had been made public. The testimony has also shown that on occasion Reagan became personally involved.

For example, in early 1985, when a Honduran military commander seized a contra arms shipment, Reagan personally telephoned the Honduran president to get the arms released, and they were.

Reagan has also acknowledged that he discussed a secret Saudi donation to the contras with King Fahd during a February 1985 visit to the White House. The Saudis were the contras' main revenue source during the aid ban, contributing $32 million between July 1984 and March 1985.

Reagan has repeatedly said that all his actions were legal.

Shortly after Reagan's role in the Saudi donation was disclosed in May, White House officials offered a new legal interpretation in which they said the Boland Amendment did not prevent the president from carrying out his constitutional responsibilities to conduct foreign policy.

Poindexter and North have argued that the fired National Security Council aide's activities were legal because they believe the council staff was exempt from the Boland Amendment. Poindexter explained that the NSC operation was carried out with the blessing of the president.

"I want to make it clear," Poindexter testified, that "the president, in effect, wanted the National Security Council staff to make sure that the contras stayed alive until we could turn the vote around in Congress and return to a program that was supported with appropriated funds."

However, Poindexter said, he could not recall Reagan specifically delegating the job to the NSC. "I don't remember a specific conversation {with Reagan} that would allow me to answer . . . in an affirmative way," the Navy rear admiral said.

Poindexter's ambiguity on Reagan's role, taken with contradictory statements by the president, has made it difficult for congressional investigators to nail down exactly what the president knew and did and whether his actions constituted a violation of the Boland Amendment.

Reagan told the special Tower review panel on Jan. 26 that "he did not know that the NSC staff was engaged in helping the contras."

Two days before the congressional hearings began, Reagan said he had "no detailed knowledge" about efforts to raise military aid for the contras.

Ten days after the hearings began, Reagan reversed himself and said, "There's no question about my being informed. I've known what's going on there . . . . As a matter of fact, I was very definitely involved in decisions about support to the freedom fighters. It was my idea to begin with."

Asked last week to reconcile Reagan's contradictory statements, White House spokesman Marlin Fitzwater declined. "They're going to stay in conflict," Fitzwater said.

In a mid-May interview with news magazines, Reagan appeared to accept the view of Poindexter and North concerning the Boland Amendment. "My interpretation was that it was not restrictive on the national security adviser or the National Security Council," Reagan was quoted as saying. ". . . And there is nothing that has ever been in the Boland Amendment that could keep me from asking other people to help {the contras}."

However, Robert C. McFarlane, who was national security adviser when the Boland Amendment went into effect, testified last week that he operated with the understanding that the measure did apply to the NSC staff. McFarlane, who was directly involved in discussions with Congress when the amendment was passed, said, "it was very evident that the intent of Congress was that this amendment applied to the NSC staff."

McFarlane said that after the amendment was enacted, he fought hard to overturn it. ". . . If we felt we were not covered, what was I doing . . . coming up here day after day trying to get rid of it," he said.

However, Poindexter, who served as McFarlane's deputy, contradicted his former boss and said he was under the impression that McFarlane had authorized North's covert contra support operation. When he succeeded McFarlane in December 1985, Poindexter said he simply instructed North to continue.

"We {the NSC} had been running this operation on our own for a long period of time because there was no alternative . . . the Boland Amendment did apply to the State Department, it did apply to the CIA and it did apply to the Defense Department," Poindexter said.

Before the ban, the administration's covert contra program had been run by the Central Intelligence Agency with help from the State and Defense departments as part of Reagan's policy of opposing Nicaragua's leftist Sandinista government.

The interpretation by Poindexter and North is based largely on their assertion that the NSC staff is not involved in "intelligence activities" and thus is not covered by language in the Boland Amendment that says the restrictions apply to any "agency or entity of the United States involved in intelligence activities."

However, Rep. Edward P. Boland (D-Mass.), the author of the amendment that bears his name, and other members of the Iran-contra panels dispute this interpetation. The NSC staff, Boland said, "is deeply engaged in intelligence activities."

Poindexter rejected assertions that he kept Congress in the dark about North's contra activities because they violated the Boland Amendment.

Poindexter said the information was withheld because he feared if North's role was revealed Congress might pass more restrictive legislation. "It was very likely that if it became obvious what we were doing, that members of Congress would have maybe tightened it up," Poindexter said.

Poindexter said that he was aware of key details concerning North's contra activities, including that North had enlisted retired Air Force major general Richard V. Secord to run a secret air resupply operation to contra forces fighting inside Nicaragua and that Lewis A. Tambs, while U.S. ambassador to Costa Rica, had helped North encourage the development of contra operations along the Costa Rican-Nicaraguan border.

The covert resupply effort, which got under way in early 1986, was halted last October after one of its cargo planes was shot down over Nicaragua, leading to the capture of crewman Eugene Hasenfus.

Poindexter said he also knew that North had recruited Felix I. Rodriguez, a former CIA operative who is a close friend of Donald P. Gregg, the national security adviser to Vice President Bush. Rodriguez testified he played a major role in assisting the covert airlift, which flew out of a Salvadoran air base.

Poindexter's disclosure that he told Reagan of the secret Costa Rican airfield, which was part of the secret airlift, raises questions about whether Reagan may have been told about the air operation.

North has testified that he sent Poindexter a photo album on the airlift with the intent that he show it to Reagan, but North said he did not know if the president had seen it.

Poindexter, who resumes his testimony on Monday, has not been asked whether he showed Reagan the photo album or whether he told the president of the covert airlift.

The president, Poindexter testified last week, "wanted the contras supported. We were reporting to him on the status of the contras in general terms, and he knew that they were surviving. And that was the thing that was important to him."