A federal grand jury in Washington has handed down a secret indictment against Tongsun Park, the South Korean businessman whose testimony is considered crucial to investigations of South Korean influence-buying in Congress.

The Washington Post has learned that a sealed indictment naming Park was filed last Friday in the court of U.S. District Court Judge John Pratt.

The nature of the charge could not be learned, but a source familiar with the evidence involving Park said it was likely to involve one of the bribery statutes, since such a violation would be included in U.S. extradition treaties with most countries.

The indictment apparently was sealed because Park is in South Korea and the United States has no extradition treaty with that country. One source close to the case added that a sealed indictment also could be used as "ammunition" to try to persuade Park to return to the United States and testify voluntarily.

Since the charges would not be made public untill the indictment is unsealed, the source said, park's agreement to cooperate might result in the indictment being dropped.

Justice Department attorneys refused to comment officially yeasterday on the details or strategy behind the indictment. But they were known to be concerned that publication of the news might harm their efforts to get Park returned to the U.S. to testify.

Both the White House and Attorney General Griffin B. Bell have said in recent weeks that they have been using diplomatic channels to ask the South Korean government for help in questioning Park.

One interpretation of the sealed indictment is that it would show the Koreans taht the U.S. government has a real need to obtain Park's testimony.

In a related development, Bell told the Los Angeles Times in an interview that the criminal investigation of South Korean lobbying had uncovered many names of members of Congress but "very few" cases that could be prosecuted.

He implied that indictments would be forthcoming on some former members but probably not against any current members of Congress.

The Washington Post reported last fall that U.S. inteligence agencies had found that the South Korean government launched an intensive lobbying campaigh on Capitol Hill in the early 1970s, using Tongsun Park as a key operative who gave cash and gifts and threw lavish parties for U.S. politicians.

The Justice Departmet and a House committee have been investigating the charges for months. Both have been actively seeking Park's testimony.

After cooperatin briefly with Justice Department investigators last fall, Park suddenly left the country for London.

He flew unexpectedly to Seoul last week, reportedly to visit his sick mother, and then held a news conferece denying that he had ever bribed any U.S. members of Congress.

He also said then that he would never return to the United States to testify before either the grand jury or the House Committee on Standards of Official Conduct.

Close Park associates have said the Justice Department has been threatening Park with indictment for months. But Justice officials recently told a briefing of members of Congress that they did not have sufficient evidence for an indictment that would fall under the extradition treatry with Britain.

Since early in the investigation, Justice Department lawyers felt confident they could indict Park as an unregistered foreign agent, sources have told The Post But that offense is no covered in the extradition treaty.

The Justice Department has come under critism for allowing Pak, 42, to leave the country in the first place and for failing to indict him earlier. Some have urged that he be indicted even if the evidence might not be strong enough for cinviction, simply as a means of getting him extradited and returned for questioning.

Justice attorneys have made it clear that they are more interested in the information Park has than in his prosecution. He has been offered immunity in exchange for his testimony, but has refused so far, according to sources familiar with the case.