The United States has asked to inspect Israel's supersecret nuclear installations to verify that American-made timing devices, apparently obtained by surreptitious means, were not used in making atomic weapons or reexported to other countries, U.S. and diplomatic sources said yesterday.

If Israel is unwilling to permit such inspection, the United States wants Israeli officials to come up with an alternative means of accounting for the estimated 500 to 600 devices.

Washington has also called on Israel to return all unused devices still in its possession because they were never licensed for export, the sources said.

The devices, tiny switches known as krytrons, provide the precise timing necessary for nuclear explosions. They are also used in oil exploration and medical equipment.

According to the sources, Israel has not replied yet to the formal requests made by the State Department. They added that the situation has potentially sensitive implications for U.S.-Israeli relations because the Israeli government was aware that U.S. policy forbids overseas sales of krytrons unless they are subjected to strict case-by-case review and licensed by the State Department.

Earlier this week, Newsweek magazine reported that a federal grand jury in Los Angeles is investigating whether the devices were smuggled out of the country in violation of U.S. law. Under the Atomic Energy Act and the Export Administration Act, violators could be liable to a maximum 20-year prison term if it is proven that the devices were exported for the national security advantage of a foreign country.

It has been widely assumed for years that Israel has the capability to make atomic weapons. However, U.S. officials have said that Israeli secrecy has prevented the United States from learning whether Israel possesses such weapons.

Israel's refusal to submit to international controls and inspection also has meant that, despite its close ties to the United States, it is barred by U.S. nuclear proliferation rules from obtaining devices, like krytrons, that could be used in nuclear weapons development.

On Sunday, the Israeli Defense Ministry, reacting to news of the grand jury probe, admitted that it had obtained a number of krytrons between 1979 and 1983 and still had a large stockpile. The ministry said the devices were used only in conventional research and development and testing equipment and that none had been sent to other countries.

Last June, a Pakistani citizen, Nazim Ahmed Vaid, was arrested in Houston on charges that he had tried to obtain 50 krytrons and smuggle them out of the country. The Pakistani government denied that his actions were connected to its reported atomic bomb program, and Vaid, after being allowed to plead guilty to a reduced charge, was deported.

Sources familiar with the Los Angeles investigation said it centers on an Israeli businessman, Arnon Milchan, who allegedly worked with a Los Angeles firm to obtain the krytrons from their American manufacturer and transfer them to Israel. The sources stressed that they could not tell whether the grand jury probe will find that U.S. laws might have been violated or that the Israeli government was involved in illegalities.

"Nevertheless," one source said, "it's clear on the face of things that the Israeli defense ministry knew that it was receiving restricted goods without having followed the prescribed rules. What remains to be seen is whether the administration or Congress will make a major issue out of this incident when it's dealing with Israeli aid or whether the matter will be soft-pedaled in exchange for an Israeli act of contrition."

Legislation recently introduced by Rep. Stephen J. Solarz (D-N.Y.) would bar U.S. aid to any country violating U.S. export laws for the production of nuclear explosive devices. However, the sources said that even if Congress adopts the Solarz amendment, it would not be applicable to the krytron incident retroactively.

Milchan, whose name is spelled Milchen by some sources, is well known in Israel for involvement in a variety of enterprises ranging from film production to international arms sales. The sources said that in 1979 he established a connection with a Los Angeles firm, Milco International, whose primary business is aviation consulting but that also engages through subsidiaries in other activities including acting as a broker for export sales.

In a telephone interview yesterday, Gretel Siler, who identified herself as corporate treasurer of Milco, said that Milchan was not among the owners of the privately controlled firm. Instead she said he had an arrangement under which he and the firm shared in the profits from export sales that he arranged.

Siler confirmed that as part of this arrangement, Milchan began in 1979 to arrange purchases of krytrons from EG&G Inc., a high-technology company based in Wellesley, Mass., that is sole manufacturer of the device.

Jack Donohue, an attorney for EG&G, said yesterday, "I don't have all the details available, but I can confirm that we sold a number of these items to that company Milco and that as far as we knew, the purchaser indicated they were for domestic use and there was no intention to export them."

Siler said she did not know the basis on which the krytrons were purchased by Milco. She referred questions to a Los Angeles attorney, James Riddet, who she said is representing Milco before the grand jury. Riddet could not be reached for comment.

Israel is the largest recipient of U.S. foreign aid. The administration has proposed aid levels for fiscal 1986 of $1.8 billion in military assistance and $1.2 billion in economic aid, as well as emergency economic assistance of $1.5 billion to be spread over two years.