In a series of increasingly testy exchanges that left him wavering between weariness and anger, former CIA director Richard M. Helms said yesterday that he still does not regard former KGB officer Yuri Nosenko a "bona fide" defector because his story about Lee Harvey Oswald is too incredible to be trusted.

The implications. Helms agreed in daylong testimony before the House Assassinations Committee are ominous.If Nosenko is lying, Helms testified, it follows that Oswald may have been "a Soviet agent when he shot President Kennedy."

"That still hangs in the air like an incubus, Helms said. He said he did not know how to resolve it short of obtaining Oswald's files from the KGB, the Soviet secret police agency.

Now rehabilitated by the CIA after some five years of imprisonment, three of them in solitary confinement. Nosenko defected to the United States several weeks after the assassination, claiming firsthand knowledge of the files the KGB maintained on Oswald in Russia. Nosenko insisted that the KGB had never even interviewed Oswald during his stay in the Soviet Union, much less recruited him as an agent.

"No person familiar with the facts finds Nosenko's statements about Oswald to be credible," Helms said. "Therefore, this tends to sour all the other opinions he maintained. I don't know how one resolves this bone in the throat."

His credibility under attack throughout the hearing. Helms denied ever labeling Nosenko as "bona fide" during his tenure at the CIA, despite a formal statement from the agency that he had done so.

Rep. Harold S. Sawyer (R-Mich.) hammered at the fact that Nosenko was paid a lump sum of $125,000 with Helms' approval in 1972, and has by now collected approximately $500,000 in consultant salaries, bonuses, resettlement expenses and other payments.

Citing what he called the "mental and physical torture" the CIA had inflicted on Nosenko in a futile effort to break him down, Sawyer suggested that the CIA had only two options: to "dispose" of Nosenko or to pay him off to keep him quiet.

"It's perfectly plain you exercised the option of paying him off," Sawyer declared at one point. "Do you dispute that?"

"Yes, I'd dispute it," Helms shot back.

"Half a million, roughly, as I add it up," Sawyer persisted.

Seething by now Helms paused, trying to control himself.

"I'm counting to 10, as my mother taught me to do," he finally said. He said the CIA merely wanted to give Nosenko "a chance to make a new life for himself" in light of the contributions he had made in other areas.

According to a CIA memorandum Helms approved on Oct. 18, 1972, "Mr. Nosenko has been an extremely valuable source, one who has identified many hundreds of Soviet intelligence officers, and he has otherwise provided a considerable quantity of useful information on the organization of the KGB, its operational doctrine, and methods . . . He has proven himself to be invaluable in exploring counterintelligence leads. He recently authored a book which is of interest to the agency.

"In effect," the memo written by then-CIA director of security Howard J. Osborn stated, "Mr. Nosenko has shown himself to be a productive and hard-working defector who is 'rehabilitated' and favorably disposed toward the agency."

The Nosenko case for years caused deep divisions within the CIA. Those in charge of him at first concluded that he was a KGB "plant," even before he arrived here from Geneva, and they put him in stringent solitary confinement on April 4, 1964, where he was kept from both the FBI and the Warren Commission, despite his stated willingness to testify about Oswald.

The so-called "hostile interogation" that ensued on and off more than three years was reportedly sanctioned on April 2, 1964, by then-Attorney General Nicholas deB. Katzenbach in a meeting at the Justice Department with CIA officials. Katzenbach, however, told the committee Thursday that he had "absolutely no recollection of Mr. Nosenko or anything to do with him in that period."

"I don't think I authorized putting anybody in jail for three years," Katzenbach said.

Challenging that account, Helms read from an April 3, 1964, memo prepared by CIA then-general counsel Lawrence R. Houston, which said that Katzenbach and other Justice Department officials had authorized "any action necessary" for the CIA to determine Nosenko's veracity and thus his status in this country.

"This was the way we handled all defectors," Helms said, at one point, of Nosenko's initial confinement. "What would you do with them?" he demanded of committee Chairman Louis Stokes (D-Ohio), "put them up in the Hilton?"

Stokes replied, "I've never been in that situation. Hopefully, I never will be."

Helms found himself under steady cross-examination, especially concerning testimony he had given the committee in an executive session on Aug 9. He assured the committee then, among other things, that Nosenko had been "put into a small house in the countryside, where he had a perfectly sanitary and satisfactory living condition."

In fact, according to the CIA, which sent spokesman John L. Hart to testify before the committee last week, the high-ranking defector was subjected to psychological torture. In an affidavit submitted to the House committee on Aug 7, Nosenko said he was at first taken to an attic room with a boarded window that kept out of fresh air and turned "very very hot in the summertime."

"I could have a shower once in a week, and once in a week, I could shave," he said. "I had no contact with anybody to talk. I could not read. I could not smoke." Later he was taken to a specially built concrete vault (at the CIA facility at Camp Peary, Va., reportedly), where even the chess sets he tried to fashion from threads of his clothing were taken away from him.

Helms swore he knew nothing about this.

LAWYER WAS INCREDULOUS: "He [Nosenko] was in your custody, and you never made any inquiry about what was happening to him?"

"Of course I did," Helms said wearily. "I was never told those details . . . that he was being kept in a room so hot he couldn't stand it. I'm genuinely sorry about it."

Nosenko was finally released for friendlier questioning, though still in confinement, in late 1967, but not until a final effort by his initial interrogators from the CIA's Soviet Russia Division to break him down. In one note made public last week, the deputy chief of that division, Peter Bagley, suggested that one option might be to "liquidate the man."

Helms said in indignant tones that he never saw the note, and never entertained the idea. He also questioned the motive of the current leadership of the CIA in making public such "lurid" details.

"It was almost as though his [CIA spokesman Hart] intention was to excoriate some of his former colleagues for their handling of the Nosenko matter," Helms said.

Hart told a reporter that the disclosures were simply "an act of honesty" on the part of CIA Director Stansfield Turner. Now retired but called back because of his expertise with the Nosenko case, Hart said Turner had told him simply to "let your conscience be your guide" in testifying about the matter.