WASHINGTON (AP) _ The FBI took out classified newspaper advertisements last summer in an unsuccessful effort to arrange a rendezvous with a man now charged with being part of a Navy spy ring, a government source says.

In the ads, the FBI tried to lure an anonymous letter-writer - who was claiming to have spied on the United States - to meetings in San Francisco or even in Mexico, and was offering to supply travel money if necessary.

The source, who declined to be identified by name, said the three ads were placed in the Los Angeles Times and were addressed to ''RUS.'' The FBI has said previously that ''RUS'' was a code name used in three letters sent to the bureau by Jerry A. Whitworth, a retired Navy radioman charged with passing secret Navy documents to the Soviet Union.

Whitworth and his attorney have denied that he wrote the ''RUS'' letters to the FBI's San Francisco office.

In 1984, ''RUS'' sent two letters to the FBI offering to expose a 20-year- old spy ring in exchange for anonymity and immunity from prosecution, the FBI has said in court papers. The first letter arrived on May 11, 1984, and was postmarked in Sacramento, Calif. The letter was dated May 7, 1984, from ''RUS, Somewhere, USA.''

The letter said ''RUS'' had been spying for several years and had passed top secret key lists used to encode military communications to a contact who turned them over to Soviet agents. This is substantially the federal charge to which Whitworth has pleaded innocent.

The ''RUS'' letter suggested using a newspaper ad to establish contact, and the first ad was placed on May 21, 1984, the source said. The ad said: ''RUS: Considering your offer. Call weekdays 9 am - 11 am 415-626-2793. ME,SF.''

The FBI has said the second ''RUS'' letter was dated May 21, 1984. That was the same day that the first ad appeared. The second ''RUS'' letter still promoted the proposal of information for immunity and noted that RUS' contact had been working for the Soviets more than 20 years and ''plans to continue indefinitely.''

On June 11, 1984, according to the source, another FBI ad appeared. This one read: ''RUS: Considering your dilemma. Need to speak with you to see what I can do. This can be done anonymously. Just you and I at 10 AM June 21st at intersection of the street of my office & Hyde St. in my city.''

That message continued: ''I'll carry a newspaper in my left hand. We will only discuss your situation to provide you with guidance as to where you stand. No action will be taken against you whatsoever at this meeting. Respond if you cannot make it or if you want to change locations. I want to help you in your very trying situation but I need facts to be able to assist you.''

Then on Aug. 13, the source said, the FBI placed an ad which said: ''RUS: Haven't heard from you, still want to meet. Propose meeting in Ensenada, Mexico, a neutral site. If you need travel funds, will furnish same at your choice of location in Silicon Valley or anywhere else.''

But, in a third letter, dated Aug. 13, 1984, the same day that the third ad appeared, ''RUS'' advised the bureau he was giving up ''the idea of aiding in the termination of the espionage ring previously discussed.''

There were no further communications from ''RUS'' and FBI checks of military personnel living around Sacramento failed to produce any suspects.

On May 20, this year, FBI agents arrested John A. Walker Jr., a retired Navy communications specialist later accused of heading a family-based spy ring that included Whitworth. The FBI has said Whitworth was identified as ''RUS'' through documents found in a search of Walker's home.

Walker, who has been described as Whitworth's closest friend, also has pleaded innocent. His brother Arthur was convicted last week of espionage and faces life in prison. His son, Michael Walker, also has been charged in the case and has pleaded not guilty.