Two diplomats who had been assigned to the Chad Embassy here apparently left the United States shortly after it was disclosed last fall that they were under investigation by federal law enforcement authorities in connection with an international heroin smuggling operation.

The U.S. Attorney's office yesterday told a federal judge that the two men, both of whom are protected by diplomatic immunity, left the country before Chad officials formally withdrew their diplomatic status and recalled them to Chad. Sources said that neither man is believed to be in that country now.

That action lifting diplomatic status late last November came after published reports linked the two Chad Embassy officials to two Pakistanis arrested on Oct. 17 by federal Durg Enforcement Administration agents. The agents recovered two kilos of pure heroin that they estimated could be cut and sold on the street for more than $28 million.

Papers filed by the government in U.S. District Court earlier this month allege that the two Chad Embassy men, first secretary Ousman Abdoul and second secretary Pascal Boulo-Ndakor, used their official positions "to facilitate the important of heroin into the United States from the Khyber Pass region of Pakistan."

According to those court papers, both Abdoul and Boulo-Ndakor flew to Pakistan with one of the Pakistanis last July. The papers said that Boulo-Ndakor received more than six kilos of heroin there and placed it in the false bottom of a suitcase.

In an interview with The Washington Post last October, Abdoul acknowledged that he had taken a trip at the invitation and expense of the Pakistanis. But he denied any involvement in illegal drug trafficking and said he took the trip because he had no work to do at the embassy at the time. Boulo-Ndakor could not be reached for comment at the time.

Meanwhile in federal court yesterday, Judge Joyce Hens Green scheduled a March 19 trial for the two Pakistanis, Hizbullah Khan, 26, and Mohammad Nisar, 23. Both men are being held in the D.C. Jail on $500,000 bond each.

A court hearing had been scheduled yesterday in anticipation of guilty pleas from both men to one count of conspiracy to import and distribute a controlled substance. The charge carries a penalty of 15 years in jail, a $30,000 fine or both.

Lawyers for both men told Green yesterday, however, that their clients wanted to stand trial in connection with the charges against them.