Federal law enforcement officials are investigating allegations that two diplomats assigned to the Embassy of Chad here participated in a drug-smuggling operation, using diplomatic cover to bring huge quantities of pure herion into the United States.

The investigation was started after agents from the federal Drug Enforcement Administration arrested two Pakistani men last Friday in a Silver Spring apartment and seized two kilograms (about 4.4 pounds) of pure heroin, which has an estimated street value of more than $28 million.

Sources close to the investigation said yesterday that law enforcement officials were told that the Pakistanis charged in connection with the drug-smuggling operation worked with two diplomats here from the Embassy of Chad, a central African nation, and an unidentified U.S. citizen in Chicago.

It was learned yesterday that the two Pakistani men, who are now cooperating with federal officials, attempted to telephone the Chicago man over the weekend but where unable to reach him.

At a hearing in U.S. District Court here yesterday, assistant U.S. Attortney Daniel J. Bernstein said the two Pakistanis had close ties with officials at an unnamed embassy here and might be given sanctuary at the embassy if they became entangled with law enforcement officials. Burnstein did not identify the embassy involved, but other sources confirmed that the diplomats allegedly involved are high ranking officials at the Chad Embassy.

Mahamat Ali Adoum, charge d'affaires at the Chad Embassy, said late yesterday he did not have complete information on the allegations, which he had read in an article in The Washington Star, and was unable to comment on the matter until he finished his own investigation. No one else at the embassy was available for comment yesterday.

Prosecutor Bernstein said at the court hearing yesterday that the two Pakistanis, Hizbullah Kahn, 26, and Mohammad Nisar, 22, allegedly have access to multiple kilograms of heroin in other areas of the United States, worth millions of dollars in street sales. Both men also have several passports and access to bank accounts containing large amounts of money, Bernstein said during a hearing before U.S. Magistrate Lawrence Margolis.

According to court records, both Kahn and Nisar live in an apartment at 4849 Connecticut Ave. NW. Bernstein, in urging an unusually high bond of $1 million for each of the men, said both have political connections in Pakistan and travel extensively in the United States. They told court officials Friday they are employed as gem dealers.

Law enforcement sources said the heroin seized Friday came from the so-called "Golden Crescent" of Pakistan, Iran, and Afghanistan, which in recent years has become a major source of heroin distributed in this country.

Bernstein said that an undercover DEA agent made a sample heroin purchase from the two men in mid-September for $50,000, and that some of that money was recovered following their arrest. According to the court records, federal agents who searched the Connecticut Avenue apartment last week confiscated $16,460 in cash, luggage, an attache case and personal papers belonging to the two men.

In preparation for the drug deal negotiated by the undercover agent for last Friday, DEA agents assembled $1 million in cash, sources said. The actual drug buy that resulted in the arrest, however, was for $600,000 worth of heroin, law enforcement sources said. According to the court records, Kahn was arrested after he showed the undercover agent a brown suitcase containing the two kilograms of heroin. Nisar, who had brought the heroin to the Silver Spring apartment from the Connecticut Avenue residence, was arrested a short time later, according to court records. a

Both Kahn and Nisar were held in custody over the weekend on the $1 million bonds. Yesterday, Bernstein said both men were "working in connection with persons who have diplomatic immunity in this country" who would be able to stop U.S. officials from bringing the Pakistanis back to court if they were released from custody. Margolis scheduled a further hearing on the case for Oct. 29. Both men are charged with conspiracy to distribute herion.

Bernstein told Margolis yesterday that the protections offered by diplomatic immunity made the case "most unusual" and "most aggravating" for the federal prosecutor's office.

A legal adviser to the State Department said yesterday that U.S. law enforcement officials are prohibited from entering embassy grounds in this country to make an arrest unless the foreign country whose embassy is involved waives diplomatic immunity.

The adviser, Horace F. Shamwell, also said that diplomats involved in criminal activity in this country can be charged only if their country's ambassador lifts their diplomatic immunity. The State Department could otherwise declare the diplomats to be "persona non grata" and seek their expulsion from the United States, Shamwell said. Such immunity is provided in the Vienna Convention on Diplomatic Relations of 1961, Shamwell said.

Shamwell also noted that customs officials cannot normally search the baggage of accredited diplomats returning to the United States. Such searches are carried out only when there is serious reason to believe that illegal items are being brought into this country, he said.