Robert K. Steinberg, the Los Angeles lawyer who said he had videotapes showing government officials participating in "sex parties," told authorities hours after they directed him yesterday not to destroy the material that the tapes had been stolen from his office.

Steinberg told the Los Angeles district attorney's office at 11 a.m. PDT that the three alleged videotapes were stolen from a gym bag in his law library, according to James Bascue, chief deputy district attorney.

Bascue said Steinberg's call came about 2 1/2 hours after an aide to the district attorney and a Los Angeles police detective contacted Steinberg and "instructed him not to destroy the tapes since they might be evidence in a trial."

The developments came a day after Steinberg gained widespread media attention by saying he had videotapes showing late presidential confidant Alfred Bloomingdale, his longtime mistress, Vicki Morgan, two Reagan administration appointees, a congressman and two businessmen participating in sadomasochistic and group sex.

Late yesterday afternoon, Bascue said Steinberg has been subpoenaed to appear in court July 25 and "to produce three videotapes involved in the Vicki Morgan murder case or to justify under oath . . . the alleged disappearance of the alleged tapes."

Los Angeles police Lt. Dan Cooke added that his department "has no substantiation of Mr. Robert K. Steinberg's public claim that a videotape exists . . . . "

Later, Steinberg said a member of the media was responsible for the disappearance of the tapes, the Associated Press reported. "Someone from the press corps went into my library this morning and took those tapes. We know which one it was, and it's under investigation," he said, according to the AP.

The events capped 24 hours of increasingly contradictory statements by Steinberg, a well-known criminal lawyer. He said an unidentified female friend of Morgan gave him the tapes because he was briefly representing Marvin Pancoast, charged with beating Morgan to death with a baseball bat last Thursday while she slept in the condominium she shared with Pancoast.

At a short, noontime news conference at his Beverly Hills office about an hour after his call to authorities, Steinberg, 46, refused to say that the tapes had been stolen and said he had no comment on that possibility.

He told reporters that after discussing the matter with White House counsel Fred F. Fielding and Los Angeles authorities yesterday morning, he could not comment on the tapes' whereabouts. He confirmed, however, that "the Justice Department, the district attorney and the counselor to the president asked me not to destroy the tapes."

On Monday night, Steinberg said on ABC-TV's "Nightline" that he planned to destroy the videotapes unless President Reagan asked him not to. White House spokesman Larry Speakes said yesterday that Steinberg called Fielding at 11:20 a.m. PDT after reporting the tapes stolen.

Steinberg told reporters at his news conference that he no longer had the tapes, and said, "I don't know where they are right now." He then reversed himself and said he knows where the tapes are. Asked if he had given the tapes to someone, Steinberg replied: "Consentually give? No."

He said other copies of the videotapes exist.

Steinberg said his wife, Elaine, called a Justice Department official and was told not to destroy the tapes, but his wife later denied making such a call.

Los Angeles police Cmdr. William Booth said that, after authorities decided to review the tapes, "Mr. Steinberg alleged that the tapes had been stolen. We advised him to contact the Beverly Hills Police Department."

Steinberg later called Beverly Hills police to ask that a crowd of reporters be evicted from his office and to report that the tapes were missing, according to police Capt. Ron Garner. He said his department is investigating the reported theft.

Steinberg told reporters Monday that he was withdrawing as an attorney for Pancoast in the Morgan murder case, in part because he did not believe the tapes should be used in the case.

Morgan, 30, last year filed an $11 million "palimony" suit against Bloomingdale, department store heir and member of Reagan's "Kitchen Cabinet." She continued to press the suit after Bloomingdale died of cancer last August at 66.

Steinberg said Pancoast's new attorney, Arthur Barens, wanted to review the tapes because Barens believed that the tapes might help explain the mental state of Pancoast, a former mental patient.

Barens, in an interview, said Steinberg had misrepresented their conversations on several points.

Steinberg had said Barens raised the possibility of using the tapes in plea bargaining with authorities but that Barens changed his mind and said the tapes should be destroyed. Barens called this "an absolute lie" and said he had no conversations with Steinberg Monday after Steinberg had promised to let him review the tapes.

Steinberg also said he did not know the identity of the friend of Morgan who gave him the tapes. But Barens said Steinberg had told him the name of the young woman.

"I have a hard time characterizing what Steinberg's motivation might be," Barens said. "I'm still skeptical that these tapes exist."

Early yesterday morning, hours before Steinberg reported the videotapes stolen, he told a reporter that he could not be sure that the tapes were in a secure place.

"How can you ever be sure that someplace is safe?" he asked, declining to say where they were stored.