D.C. Superior Court Chief Judge Fred B. Ugast ordered yesterday that Tran Van Khiem undergo more psychiatric tests to determine whether he is competent to stand trial on charges that he murdered his elderly parents, who were former top South Vietnamese diplomats.

Khiem's parents, 88-year-old Tran Van Chuong and 75-year-old Nam-Tran Chuong, were found July 24, 1986, suffocated in their bedroom at the Northwest Washington home they shared with their son.

Khiem, 61, was indicted in April on two charges of first-degree murder. If convicted, he would receive a prison sentence of 20 years to life.

Khiem's father had been the South Vietnamese ambassador to the United States, and his mother had been the country's permanent observer to the United Nations. They resigned in 1963 in protest of the South Vietnamese government's treatment of Buddhists. The couple remained prominent within the local Vietnamese community.

One of Khiem's sisters is Madame Ngo Dinh Nhu, who was the sister-in-law of assassinated South Vietnamese President Ngo Dinh Diem and who was known as the Dragon Lady during the Vietnam War.

Most of yesterday's brief court hearing was held at the judge's bench, out of the hearing of the public and press. At the end of the hearing, Ugast said he was not satisfied with the report of a mental competency examination conducted by a St. Elizabeths Hospital physician.

That report, Ugast said, was "stated in strictly conclusory terms without any discussion of the underlying examination."

Although the report said that Khiem is competent to stand trial and that he understands the charges and can help his lawyers in his defense, Ugast said he was troubled by an apparent disagreement between Khiem and defense attorney Thomas Mauro over use of an insanity defense.

Ugast also said he wanted Mauro to assist in the competency determination by participating in an interview with psychiatrists. A summary of the interview would not be available to prosecutors but would be used to determine whether Khiem can communicate with his lawyer.

In addition, Ugast said he might consider appointing an independent lawyer to present an insanity defense even if Khiem decides not to allow his own lawyer to do so.

Khiem waived his right to an insanity defense in May, and at a hearing in June, he told Ugast he thought there was a conspiracy forcing him to plead insanity against his will.

Khiem, who was a lawyer in Paris and a paramilitary official in South Vietnam, has denied killing his parents, saying they died from falls.

Last month, a D.C. Superior Court judge blocked the sale of the couple's house after the estate's administrator found a buyer. Khiem is also contesting his parents' wills, in which they disinherited him.

Ugast said he wanted the reports from the additional tests returned within 30 days, and he scheduled another competency hearing for Sept. 18.