Despite repeated U.S. protests, Switzerland has been knowingly exporting to Pakistan sophisticated nuclear technology that Pakistan is believed to be using to develop an atomic bomb.

The Swiss government, which contends it is violating no international agreements, has allowed several Swiss firms to sell Pakistan vital components for a gas centrifuge process to produce highly enriched uranium necessary for nuclear weapons.

The Swiss technology has helped enable Pakistan to continue development of this process despite steps taken by the United States and some European countries to curtail such exports to Pakistan.

In the latest of a series of secret diplomatic complaints dating back to last year, the U.S. State Department informed Switzerland a few weeks ago that it had evidence that five Swiss firms are still providing equipment and technical assistance for the centrifuge process being developed by Pakistan at a well-guarded plant by Kahuta, about 25 miles south of Islamabad, the capital.

The Carter administration has told Switzerland this is undermining its efforts to prevent Pakistan from developing nuclear weapons. U.S. officials fear that would dangerously expand the nuclear arms race in the developing world and create the possibility of nuclear war between Pakistan and India, which exploded a nuclear device of its own in a 1974 test.

The specific items being exported to Pakistan by Swiss firms appear on no international list of materials barred from such transactions, the Swiss contend, largely because they can be used for a variety of purposes other than nuclear technology.

At the same time, the United States, among other countries, has shown itself willing to depart from antiproliferation guidelines when it considers this politically useful.President Cater, for example, has overruled the Nuclear Regulatory Commission and pressed for shipment of low-enriched uranium to India, although the NRC ruled the transaction in violation of the 1978 Nuclear Nonproliferation Treaty. This issue is awaiting congressional action.

Swiss officials acknowledged here that they knew of the activities of several if not all of the firms named in the latest formal complaint from Washington. But they said the Swiss government has not acted to stop the exports to Pakistan because it does not believe they violate either Swiss law or international agreements to stop the spread of nuclear weapons.

Swiss officials also suggested that some of the components being exported by Swiss firms to Pakistan may be earmarked for another method of producing nuclear material for a bomb: a reprocessing plant to separate weapons-grade plutonium out of used nuclear fuel from the Canadian-built Pakistan nuclear reactor that generates electricity for Karachi. But they said exports of these components also apparently were not covered by Swiss law or international guidelines.

"We wish to analyze the information we got from our American friends to see if anything got through our controls," said Switzerland's new deputy foreign minister, Raymond Probst, who was leaving Washington after more than four years as Swiss ambassador when he received "this new information" from U.S. Assistant Secretary of State Thomas Pickering.

"We will explain to our American friends what the situation is," Probst said in interview here. "We are aware of the importance of the whole thing. We know we have to clarify it for the United States and for ourselves because we believe in nonproliferation and don't want to help the spread of nuclear weapons."

Nevertheless, some U.S. officials believe Switzerland is knowlingly violating the spirti if not necessarily the letter of the 1968 nonproliferation treaty, signed by Switzerland and 113 other countries, as well as a supplementary agreement by a "suppliers' club" of 15 exporters of nuclear technology, including Switzerland, to restrict transfers of "sensitive" technology that could be used to produce nuclear weapons.

"What the Pakistanis are getting from Swiss firms is very important," said one informed U.S. source, "not in terms of sensitivity, but it is high-quality precision equipment that they would have difficulty buying elsewhere and certainly great difficulty manufacturing on their own."

Swiss officials answered that the Swiss exports to Pakistan are not prohibited by either the suppliers'club agreement or Swiss law because they are components that can be applied to a wide variety of uses.

The international agreement and Swiss law control the export only of a list of specific processes for producing weapons-grade nuclear materials, including the gas centrifuge process being developed by Pakistan, but not their individual components, Swiss officials pointed out.

"If it is not on the list, we cannot control its export," said Herbert von Arx, the Swiss Foreign Ministry's expert on nuclear and legal matters. "Unfortunately, some things are not on the list, but Switzerland is a free country. We have told companies you do not have to inform us about what is not on the list."

Switzerland has refused repeated U.S. requests to close what Washington considers this loophole -- as the United States, Canada, Britain and some other countries have -- either by adding the specific components to the Swiss list of "sensitive" technology whose export should be controlled or by restricting the export of anything the Swiss government believes is likely to be used by Pakistan to develop nuclear weapons, whether it is on the list or not.

Probst said the Swiss government would not do this by itself because it would unfairly handicap Switzerland's nuclear technology industry in competition with other nations that, the Swiss say, also are not strictly limiting their exports in the way the Carter administration wants.

"It's just not possible" for Switzerland to do what Carter administration asks, Probst said. "We will not do it."

However, he said he has told U.S. officials that Switzerland is willing to discuss new controls that would be binding on all nations exporting nuclear technology so Swiss industry "can compete on an equal basis."

"We wish to maintain our highly developed nuclear technology industry," Probst said. "The rules should be the same for everyone. Everyone should have the same chance. This is in keeping with the American philosophy. We appeal to something very American."

Probst and other Swiss officials also suggested that stricter controls by Switzerland's relatively weak federal government might risk a rebellion by Swiss industry, which has opposed ratification of the Nuclear Nonproliferation Treaty by traditionally neutral, free-trading Switzerland.

"Our industry felt the nonproliferation treaty might be a hindrance to industrial activity and exports in the nuclear fields," Probst said, "but the diplomats convinced them to go along."

Probst said there had been Swiss trade with Pakistan in specialized nuclear technology for some time before "we were informed by the United States in 1979 of its worries about the development of a nuclear industry in Pakistan because it might lead to the development of nuclear weapons."

"It's everybody's guess that this is what Pakistan is doing because of the Indian nuclear explosion," Probst said"But this is not the official view of the Swiss government. Switzerland had not made a judgement."

The Swiss government was first asked by the United States in 1979 about exports to Pakistan by two Swiss firms, called VAT and KORA, of essential components for the gas centrifuge process being developed at Kahuta. Oversimplified, the process is designed to enrich uranium by rapidly rotating uranium hexafluoride gas in a series of hundreds of centrifuges, separating heavier uranium-238 from lighter uranium-235.

VAT sold Pakistan vacuum valves to be used to regulate the stream of uranium hexafluoride gas into and out of the centrifuge system, according to Claude Zangger, the scientist in charge of nuclear techology export policy and controls for Switzerland's federal office of energy here.

"But these valves are not limited to this application," Zangger said. "They are not on the international or Swiss list" of sensitive technology whose export is to be restricted.

"VAT is known all over the world for the quality and efficiency of its vacuum valves," Zangger added. "It has been selling them for the past 30 years to all kinds of laboratories for things like accelerators and vacuum technology."

Nevertheless, VAT asked the Swiss government if an export license would be needed because of the values' use in a gas centrifuge uranium enrichment process. The firm was told by the government that do license was needed because the vacuum valves themselves were not on the suppliers' club list, although the gas centrifuge process was.

KORA specially designed, built and supplied to Pakistan in 1978 and 1979, according to Zangger, both an evaporation system for converting the uranium hexafluoride into a gas at the beginning of the centrifuge process and a condensation system for recovering the uraniumn hexafluordie as a solid at the end of the centrifuge process.

Zangger said KORA must have known what these systems were being custom-made for, and KORA also checked with government officials before exporting them to Pakistan. But Zangger added that "it is an operation made hundreds of times in the chemical industry" and also was not specifically listed among the technology restricted by either the suppliers' club guidelines or Swiss law.

Probst said Switzerland "made it clear" to U.S. officials last year that the technology VAT and KORA were supplying to Pakistan was not on those lists and that its export was perfectly legal and could not be stopped by the Swiss government.

"He thought it was understood," Probst added.

But U.S. officials kept pressing Switzerland to halt the exports. Then, as Zangger put it, "another wave of new information from the U.S. authorities" hit the Swiss government in the last few weeks.

"We feel there is a lot of misunderstanding in the new information," Probst said, referring to the reappearance of VAT and KORA among the five Swiss firms named by U.S. officials in the new complaint. "We feel a lot of it goes back to a year ago when we thought we had it under control."

"The Americans say that KORA people are still there" at the Kahuta site in Pakistan where the gas centrifuge process is being developed, according to Zangger, who said he believes "KORA may now be involved in after-delivery service there" of the systems it sold to Pakistan.

Zangger said he was surprised that the new U.S. information stated that "there were also additional exports by VAT" to Pakistan.

Zanger said he had been told by VAT officials that they would not sell anything more to Pakistan because of "bad publicity" in Switzerland. Leaks to the press, which Zangger blamed on "many indiscretions from Washington" based on last year's U.S. complaint to Switzerland, led to Swiss newspaper headlines that said, according to Zangger, "Swiss Firms Helping Pakistan Build Bomb."

Zangger and Probst said the new U.S. complaint also identified three other Swiss firms -- whose names they would not disclose -- who it stated were exporting a ventilation system, aluminum tubing and machine tools to Pakistan for use in the uranium enrichment project. Zangger said he would be discussing all this information with the firms involved.

"We feel Washington has been told about equipment that has no application at all to nuclear weapons," Probst said. "For example, air conditioning installation. If it happens to be used in reprocessing plants to cool people, we believe we can't control it."

Zangger said Probst was referring to a special ventilation system that Zangger thought might be for use in reprocessing plant for the separation of weapon-grade plutonium from used nuclear reactor fuel -- the other possible Pakistani route to a bomb. Zangger said U.S. officials may be mistaken in thinking the ventilation system was to be used in the uranium enrichment process.

"The Swiss government was asked some time ago if [the special ventilation system] fell on the first list under reprocessing" of restricted nuclear technology exports, according to Zangger, who said the firm was told the item was not on the list and no export license was needed.

Citing the "machine tools" he said also appeared in the new American information, Probst said. "They're used for everything. If it happens to be used in a plant for nuclear fuel production, it does not by itself have anything to do with nuclear fuel production."

Swiss Foreign Office legal expert Von Arx, who was sitting in on the interview, asked, "What about windows? What if they were being exported to Pakistan?"

"Or furniture?" added Probst.

"You have to draw the line somewhere," Von Arx said.