The State Department yesterday abandoned its low-key approach to the case of two American newsmen threatened with slander charges in a Soviet court in Moscow and warned Soviet authorities to "reflect very carefully on the broader implications of this issue."

The U.S. statement was the first threat of retaliation - albeit veiled and indirect - since the journalists were first notified Tuesday that they had been accused of slander.

The two newsmen - Craig R. Whitney of The New York Times and Harold D. Piper of The Baltimore Sun - have been accused of defaming "the honor and dignity" of Soviet television employes who resented stories both wrote on a televised confession by a convicted Soviet dissident.

Both men filed stories on May 24 reporting that some friends of the dissident, Zviad Gamsakhurdia, believed that the authorities had falsified a filmed "confession" from him broadcast on Soviet television.

The two Americans face a trial July 5 on charges that these reports were slanderous. If found guilty, under Soviet law they would have to publish a retraction or pay a fine.

The episode highlights the unusual role played by western correspondents in Moscow as conduits of information about Soviet society to Soviet citizens, despite the fact that they are writing or broadcasting for media out-lets in the West.

This occurs because western news reporters are picked up and rebroadcast to the U.S.S.R. in Russian by the Russian-language services of the Voice of America, British Broadcasting Corp., West German radio and Radio Liberty, based in Munich.

For Example, the stories by Whitney and Piper were both mentioned in a program on the VOA's Russian Service summarizing American press reports from the Soviet Union.

Radio Liberty also reported on Whitney's dispatch, though not piper's.

VOA made the script of its broadcast on the stories available yesterday. The broadcast contained abridged sections from both reporters' stories.

It noted that both stories said friends of Gamsakhurdia believed the filmed confession was "a fabrication." Voa then quoted Whitney's story on the comments of the dissident's wife, who disputed the confession, and Piper's admittted speculation that the confession might have been a montage of filmed statements by the dissident.

The two stories did not cover exactly the same ground. Whitney did not report the "montage" idea, but said Gamsakhurdia had told a friend who visited him in jail the day after it was broadcast, "I don't know when they could have filmed it."

Whitney also reported an alleged confession from Gamsakhurdia published in a newspaper in Tbilisi, his home town, the capital of Soviet Georgia. Whitney quoted the editor of the paper as saying the confession was sincere.

Piper's story did not mention the published confession.

The broadcast accounts of these stories brought to Soviet listeners a type of news they never see or hear in their own media - an allegation from Soviet citizens that the Soviet authorities doctored an attempt to discredit political dissidents.

Without the radio to bring that embarassing idea back to Soviet citizens' ears, it is unlikely that Soviet authorities would have taken action against Piper and Whitney.

Even with the existence of radio broadcasts for nearly 30 years, this accusation of "slander" is an unprededented event Soviet sources in Washington have said the step was taken because the Soviet government feels it must do something to stop western correspondents from filing so many stories on dissidents and other "negative" apects of Soviet life.

These sources said a study had been made comparing the stories written by western correspondents in 1967 and 1977, and the study concluded that the stories have become much more critical of the Soviet system.

Ten years ago the western press had just become aware of the dissident phenomenon, and reporters in Moscow had few direct contacts with the early representatives of those Soviet citizens who began actively to demand liberalization or to protest government actions.

In recent years western reporters have maintained extensive and regular contacts with dissidents, writing much more about their activities. Re-broadcast of their stories on the Russian broadcasts of western radio stations had made the Soviet public much more aware of the dissidents much more aware of the dissidents in its midst than otherwise would have been possible.

The officially controlled Soviet media have only mentioned the dissidents to attack them.

The Soviets singled out Piper and Whitney, but the Moscow correspondent of the Paris newspaper Le Monde, Daniel Vernet, filed a similar though shorter story that appeared in Paris five days before the Whitney and Piper stories appeared in this country.

Le Monde said the televised confession "appeared to have been cut" when broadcast, and raised the doubts of friends that there really had been a confession.

State Department spokesman Hodding Carter yesterday declined to elaborate on the "broader implications of this issue" mentioned in the State Department statement. "They speak for themselves," Carter said. "Our concern has to do with the ability of our journalists to function freely."

For many years, the U.S. government has taken tit-for-tat retaliatory action whenever the Soviet authorities have disciplined or expelled an American correspondent.