U.S. Attorney Earl J. Silbert last year agreed with the Chilean government to limit the amount of information the Justice Department would release about the assassination of former Chilean ambassador Orlando Letelier and other activities allegedly involving the Chilean government, it was disclosed yesterday.

Silbert's April 7, 1978, agreement with Enrique Montero Marx, the Chilean under secretary of the interior, came one day before the Chilean government turned over one of its secret police agents, Michael Vernon Townley, to the FBI for questioning in the Letelier slaying.

Townley, who since has confessed to planting the bomb in Letelier's car that killed the former diplomat and a colleague, Ronni K. Moffitt, is the government's chief witness in the U.S. District Court trial here of three Cuban exiles charged in connection with the Sept. 21, 1976 Letelier killing.

Townley, after being released from his secrecy vows with the secret police, once known as DIAN, agreed last April 17 to plead guilty to conspiring to murder a foreign official in exchange for several concessions from the U.S. government. Townley, under the April 17 agreement, does not have to testify about his DINA activities that do not involve U.S. citizens or did not occur in the U.S.

In addition, Townley eventually will be sentenced to a 3 1/2-to-10-year sentence and the government has agreed to recommend that he be paroled after serving 40 months.

The disclosure of Townley's DINA activities, aside from the Letelier killing he says DINA ordered him to commit, has been a repeated point of contention in the trial of the Cubans, Guillermo Novo Sampol, his brother, Ignacio Novo Sampol, and Alvin Ross Diaz.

U.S. District Judge Barrington D. Parker has ruled, over the objections of defense lawyers, that they cannot question Townley about his alleged role in the 1974 assassination of a prominent Chilean exile and his wife in Buenos Aires and the attempted killing of another exile and his wife in Rome a year later. In addition, the judge refused yesterday to allow extensive questioning of Townley about a 1975 European DINA trip the said he took.

At a bench conference yesterday, Assistant U.S. Attorney Eugene M. Propper told Parker that the Silbert agreement committed the government to only use "any information we had" in U.S. courts or give it to Chile. "That is," the prosecutor said, "we would not spread it to the press or give it worldwide" and that Chile would give information it had in the Letelier case to the U.S.