Assistant Secretary of State Elliott Abrams, the administration's controversial "point man" on Nicaragua since 1985, admitted yesterday that he had misled Congress on several occasions, but asserted that he had been repeatedly deceived by former White House aide Oliver L. North and other officials about their secret support of the contras during a ban on U.S. military aid.

Abrams, who had given Congress categorical assurances that there was no improper U.S. government involvement in the contras' war in Nicaragua, yesterday gave the House and Senate investigative panels an account that was often at odds with testimony from previous witnesses in what is now the fifth week of the Iran-contra hearings.

For example, Abrams disputed sworn testimony from former U.S. ambassador to Costa Rica Lewis A. Tambs, retired Army major general John K. Singlaub, and the contents of a memo addressed to him by his own deputy, Richard H. Melton. He also dismissed as "puffery" an Aug. 26, 1986, memo from Carl R. (Spitz) Channell, in which the Washington fund-raiser credited Abrams with helping in a 25-state educational and lobbying campaign to rally support for the Reagan administration's Nicaraguan policy.

Time and again Abrams demonstrated that he had -- as Senate counsel Mark A. Belnick put it -- a "way with words." He agreed with Belnick's assertion that his approach to testimony before congressional panels in the past meant that "unless the senators asked exactly the right question, using exactly the right words, they weren't going to get the right answer."

Nevertheless, the contentious collision between Abrams and the investigators that many observers expected never came in the first day of his testimony. Committee sources insisted, however, that the six hours of questioning by three committee lawyers set the stage for members to probe more deeply today into Abrams' past performances before Congress, and to explore numerous contradictions that arose yesterday between his version of key events and those of other witnesses.

Yesterday's questioning also turned up the fact that Secretary of State George P. Shultz was concerned enough about Lt. Col. North's activities on the staff of the National Security Council in September 1985 to direct Abrams to "monitor Ollie," whom Shultz called a "loose cannon."

Abrams disputed the "loose cannon" description as a "bum rap." During the next 15 months, he acknowledged, his only "monitoring" consisted of asking North several times if he were violating the law.

Abrams said he never reported to Shultz that North was aware of a plan to build a secret airstrip in Costa Rica for the use of the private airlift supplying the contras.

In one of the few barbed exchanges of the day, Abrams also said he had never asked North directly if the White House aide had any involvement in the flight of the C123K cargo plane shot down in Nicaragua last Oct. 5. In fact, other testimony has established that North helped organize and direct the private airlift of which the plane was a part.

After emphasizing that Abrams had taken the lead in telling Congress and Shultz that there was no U.S. role in the resupply operation, Belnick suggested that Abrams did not ask North "point blank" because "you were afraid that you'd get the wrong answer."

Abrams denied that. But under further prodding he admitted that by not getting the facts from North, he had subsequently made false statements to Congress, the public and the press.

Abrams gave the committees a sample of his legendary testiness a few minutes into the hearing yesterday when Belnick asked him about Tambs' assertion that a secret, three-member committee had directed the creation of a contra "southern front" in Costa Rica.

The Restricted Interagency Group (RIG) described by Tambs was chaired by Abrams and included North and the Central Intelligence Agency's Alan D. Fiers, head of the agency's Central American task force.

Asked to respond to Tambs' claim, Abrams retorted: "Well, he doesn't know what he's talking about -- and he never attended a RIG meeting."

Abrams at first claimed that the RIG was actually a large interagency group with 15 members. Belnick then noted that Abrams' desk calendar showed seven meetings of the triumvirate in the first half of 1986, and only four meetings of the larger entity.

Abrams acknowledged that the RIG had been restricted: he said he had "barred the door" to other State Department entities, including the Bureau of Intelligence and Research, the senior deputy assistant secretary of which Abrams considered to be "biased" against Reagan administration policy in Nicaragua.

Abrams' testimony also clashed with Tambs' on another central issue touching on the Reagan administration's support for the private airlift to the contras: the matter of the U.S. role in creating a fighting force of anti-Sandinistas in southern Nicaragua.

Tambs said last week that North had assigned him to open a "southern military front" and that he had subsequently discussed the matter privately with Abrams during a September 1985 meeting of U.S. ambassadors in Panama, a discussion that Abrams yesterday denied having had.

Concerning a secret airstrip built with private funds in northern Costa Rica to help the contra airlift, Abrams denied knowing that any U.S. government officials had been involved in the project.

"It was pretty clear from the way it was told to me that no U.S. officials were involved," he said. "That would have been illegal."

Abrams insisted repeatedly that the airstrip was being constructed strictly by the contras' "private benefactors." He said it was comparable to hundreds of other small strips in the region.

Abrams said he never asked North about it, and that the first time he knew that Tambs had helped negotiate permission to build it with Costa Rican officials was when Tambs testified last week.

"He never reported that to the Department of State," Abrams said, "and it would never have occurred to me that an ambassador would do something like that without checking with the department."

Abrams also acknowledged that he never asked Tambs to check into it, even though it was obvious on a trip Abrams made to Costa Rica that the airstrip was a matter of great sensitivity.

The local CIA station chief, he said, almost had a "cardiac arrest" when Abrams brought up the matter. The assistant secretary said he could not remember how he had heard about it.

Later, while still insisting that the airstrip was a private operation, Abrams told how, in September 1986, he had directed Tambs to deliver an "implied threat" to Costa Rican President Oskar Arias Sanchez unless Sanchez agreed to cancel a proposed news conference disclosing the airstrip's existence.

Abrams recalled directing Tambs to "make it clear to President Arias that his visit {to the United States} was at risk."

A major part of Belnick's examination yesterday focused on the extent of Abrams' knowledge and involvement in fund-raising with foreign countries on behalf of the contras.

To begin with, the State Department official said he had no knowledge of the $32 million in Saudi Arabian contributions during 1984 and 1985 that enabled the contras to double in size and buy $19 million in arms when all U.S. government assistance was cut off.

Abrams went out of his way to distance himself from the private efforts to raise money for the contras. He said, for example, that he never promised to give assistance to Singlaub's efforts to help raise money from Taiwan and South Korea despite the general's sworn testimony that Abrams did.

He denied the accuracy of a memo from his deputy reporting on an earlier commitment to Singlaub.

But Abrams said that he had, in fact, misled several congressional committees last fall in the matter of his own solicitations.

Several hours after declaring in his opening statement that "Congress must be a powerful participant in the making of foreign policy," Abrams admitted that he had made a "great mistake" last Nov. 25 when he misled the Senate Select Committee on Intelligence about his solicitation of $10 million for the contras from Brunei.

With Shultz's authorization, Abrams said, he went to London in August 1986, checked into a hotel and met a Brunei representative in a park after identifying himself as "Mr. Kenilworth."

In the silvan security, Abrams suggested a contribution of $10 million to the contra cause.

When the contact asked what "concrete" benefits Brunei would receive, Abrams said Brunei would have the "gratitude" of the president and secretary of state.

The U.S. ambassador in Brunei cabled the department in September that the $10 million request had been granted and that, as a token of appreciation, "I thought {the sultan} would enjoy the program that had been arranged for him" aboard a U.S. aircraft carrier, and "was very much looking forward to the experience."

The State Department vetoed the idea of having the money sent into a CIA-arranged account and directed the sultan to deposit it in a Swiss account, the number of which was provided by North's secretary, Fawn Hall.

In the major concession of the day, Abrams acknowledged that he had made a "great mistake" in failing to disclose his role and in misleading the Senate intelligence committee about the existence of any solicitation.

He was, he said, "caught in a bind" because he did not have Shultz's authorization to disclose the Brunei solicitation.

House chief counsel John W. Nields Jr. suggested, however, that the real reason for Abrams' misleading remarks was his fear that the disclosure would lead to the Swiss bank account -- the same one into which funds from the U.S. arms sales to Iran were channeled in 1986.

Abrams responded, "Gee, I don't know, I suppose so."