A federal grand jury in Los Angeles indicted Richard K. Smyth, owner of a small electronics firm, yesterday on charges that he illegally exported to Israel 800 sophisticated timing devices that can be used to trigger nuclear bombs.

Smyth, 55, operator of Milco International Inc. of Huntington Beach, Calif., was charged with 15 counts of violating the Arms Export Control Act and 15 counts of making false statements to disguise the shipments.

If convicted, he faces a maximum of two years in prison and a $100,000 fine for each export act violation, and five years in prison and a $10,000 fine on each false-statement count.

Announcement of the indictments came four days after the Israeli Defense Ministry, reacting to news of the grand jury probe, admitted that it had the devices, known as krytrons.

The tiny switches, which provide the precise timing necessary for nuclear explosions, are also used in strobe lighting, oil exploration and medical laser equipment.

Robert C. Bonner, the U.S. attorney in Los Angeles, refused to comment on questions about whether the Israeli government had been involved in the alleged illegal actions.

State Department spokesman Edward Djerejian said, "I can only note that the indictment does not mention any Israeli citizen."

Djerejian added that the United States "has expressed its serious concern to the Israeli government about this alleged violation of U.S. law" and had been assured that Israel will cooperate with the continuing U.S. investigation "to the full extent permitted under Israeli law."

He also said Israel had informed the United States that the krytrons "were used for nonnuclear applications in defense-related programs such as laser range-finders" and would provide Washington with "a formal written assurance that the devices will not be used for nuclear purposes."

"I can also confirm that law enforcement authorities did request that the krytrons remaining in stock in Israel be returned, and the government of Israel has agreed to do so," Djerejian said.

The spokesman said he could not comment on reports that the United States has asked for a full accounting of how and where the krytrons had been used. However, other U.S. officials, who asked not to be identified, said it could be inferred that such an accounting will be made.

U.S. policy forbids overseas sales of the krytrons unless they are subjected to strict case-by-case review and are licensed by the State Department.

The indictment said Smyth made 15 shipments to Israel, beginning in 1980, and circumvented the licensing requirement by preparing false documentation to cover the shipments.

According to the indictment, the shipments were made to Heli Trading Ltd., an Israeli firm reportedly owned by Arnon Milchan, an Israeli businessman.

Although Milchan is not mentioned in the indictment, a Milco employe, Gretel Siler, told The Washington Post earlier this week that he had been associated with Milco in various export transactions and had been involved in purchasing the krytrons from the manufacturer, EG&G, of Wellesley, Mass.

Sources familiar with the investigation said last night that at least one agent of the U.S. Customs Service has been in Israel investigating Milchan's connection with Milco. Siler also told The Post that Milco's principal business is consulting in avionics, which involves the miniaturization of electronic equipment for use in airplanes.

She said Smyth has been a consultant to the U.S. government, but U.S. officials said they could not confirm that.

Smyth reportedly has been traveling in Europe. Justice Department officials in Los Angeles said arrangements had been made for him to be arraigned in U.S. District Court there on May 28.