Former White House counsel Fred F. Fielding said yesterday that he cautioned an associate of Michael K. Deaver that Deaver's lobbying firm should curtail efforts to contact senior White House officials "regardless of the technicalities of the law."

Fielding said he assumed that his warning to William Sittman was relayed to Deaver, who had resigned as White House deputy chief of staff a month earlier in 1985 to form a lobbying firm here.

Fielding's testimony highlighted a central element in the five-count perjury case against Deaver, who has maintained that his lobbying contacts with senior administration officials were legal and proper under a ruling in 1983 by the Office of Government Ethics.

Despite the ruling, Fielding said, he did not want Deaver or Sittman, both of whom left the White House in May 1985, contacting senior officials.

Fielding said he suggested that they could talk to Secretary of State George P. Shultz or National Security Council (NSC) staff members because such contacts would be acceptable under the ethics ruling.

At one point, Fielding said, he told Sittman that he was willing to go to Georgetown University Hospital, where Deaver was a patient, to spell out his concerns.

Deaver, who essentially is charged with failing to recall his contacts with administration officials in congressional and grand jury testimony, apparently did not share Fielding's concerns about the contacts.

Fielding said Deaver called him at one point and pleaded for permission to talk to national security adviser Robert C. McFarlane, arguing that it would be "in the best interests of the United States." Fielding said he rejected that and told McFarlane and Deaver that they should not talk.

McFarlane has testified that Deaver called him to discuss a tax benefit for Puerto Rico, a client of Deaver's firm, but McFarlane said that he canceled a proposed meeting with Deaver after Fielding advised against it.

Deaver's contact with McFarlane is one of those that he allegedly failed to recall in his grand jury testimony.

Fielding, the 47th witness at Deaver's trial in U.S. District Court, said he had discussed with Deaver on five occasions what contacts Deaver could make after leaving the government. Included, he said, was one session before Deaver left the administration.

On the 17th full day of testimony, the courtroom was so quiet as the husky, silver-haired Fielding sat in the witness chair that the squeaking of Judge Thomas Penfield Jackson's leather chair was easily audible.

Prosecutor Whitney North Seymour Jr., moving to rest his case perhaps as early as Thursday, also produced another former White House aide who said Deaver contacted him.

William F. Martin, now deputy energy secretary, said that at a luncheon shortly before lobbyist Deaver left for South Korea in 1985, Deaver "mentioned that it would be nice to have a letter of introduction" from President Reagan to South Korean President Chun Doo Hwan.

Martin, Deaver and Sittman were at the lunch, Martin said. Deaver was "looking out the window" at the time of the remark, and there was a pause in the conversation, Martin said.

The remark "wasn't directed at me because I . . . didn't have the power to get the letter," said Martin, noting later that there were "rigorous rules" specifying who could provide such a letter.

Richard L. (Dixie) Walker, former U.S. ambassador to South Korea, has testified that Deaver pulled a letter from Reagan from his pocket and handed it to Chun during their 1985 meeting.

No other witness has supported Walker's account. Martin joined them, saying he found no record of it in NSC files that he supervised at the White House.

Martin said Deaver asked him about U.S. policy toward Korea "so he wouldn't put his foot in his mouth" during his visit. Deaver, who was going to Seoul on behalf of the Philip Morris tobacco company, never mentioned his client during the luncheon, Martin said.

Andrew Singer, a Washington tax-law specialist, provided one of the day's few light moments when he recalled that "he was impressed" that Deaver declined to interrupt a meeting on the Puerto Rican tax issue to take a telephone call from First Lady Nancy Reagan.

Asked if he typically stopped his business meetings for telephone calls, Singer replied: "If Mrs. Reagan called, I would."