A State Department official maintained under more than four hours of cross-examination yesterday that about 100 U.S. diplomatic cables allegedly stolen by an espionage ring seriously breached the national security, compromised secret sources and tipped off the Vietnamese to U.S. diplomatic strategy.

Defense attorneys for Ronald L. Humphrey, the United States Information Agency official accused of stealing the documents, and for David Truong, the Vietnamese expatriate who is accused of passing the documents to a government informant, attacked Kenneth M. Quinn's statements throughout the fifth day of the Vietnamese espionage trial in Alexandria.

But Quinn never retreated from his testimony.

The government has charged that the stolen cables were delivered by a courier to the Vietnamese in Paris, and Truong and Humphrey are accused of espionage, conspiracy and stealing government documents. The sensitivity of the document, however, is an important factor in the case because the defense claims the documents contained only "diplomatic chitchat" and other meaningless information.

Defense attorneys contended in U.S. District Court in Alexandria that most of the information in the cables had already been made public and should never have been classified as top secret and confidential in the first place.

And under questioning, Quinn conceded that the information in most of the documents had already been made public.

But, he said, disclosure of any classified material - even material that is publicly known - is dangerous because it violates agreements made with sources who provided the information because it reveals information given in confidence about other countries and because it gives U.S. enemies insight into the government's bargaining strategies.

The danger of disclosing a source, Quinn testified, is revealed in cables that merelly state the widely reported assertion that Soviet relations with Vietnam were deterlorating. The cables identified the source of the information only as "a Third World source living in Hanoi." But "bits of information in the cable narrowed the possibilities" of who the source could be to "probably an Indian," Quinn said. He testified that there the informant might be identified.

Quinn's major point, repeated many times, was that the casual and chronic release of classified information by sources around the world, would cause those sources to dry up.

"What about diplomatic gossip?" asked Humphrey's defense attorney Mark L. Foster. "Like the cable which remarks about how the Finnish ambassador is probably henpecked by his wife?"

"That type of information, while seemingly innocuous, is used by us in biographical data, if they're strong people, weak people, the effects their wives have on them," Quinn responded.

"Then the Finnish ambassador sees it and says 'that's what the Americans think of us, then I won't talk to them anymore.'"

Foster asked Quinn about a cable saying that U.S. diplomats complained because upper-echelon Vietnamese officials did not attend an American cocktail party. Quinn said the cable was important because it showed that lower-level Vietnamese officials did attend and that was a sign of improvement of relations between the two countries.

Foster gave another example. One cable quoted a Vietnamese official complaining of the increased number of prostitutes in Saigon.

"The statement . . . in and of itself would never be classified, as far as I know," Quinn answered, smiling. But he said the revelation of the statement's source would probably discourage that source from giving the U.S. information again.

"Prostitution in the streets is a subject that's quite sensitive to the Vietnamese," Quinn said.

Truong's attorney, Michael E. Tigar, pounded away at Quinn's testimony by asking the state department official why the information in dozens of cables breached national security.

Tigar was trying to make the point that government documents May be over-classified.

Quinn failed, however, to raise to the bait. "It's my assumption, he said," that (classified documents) are always properly classified."