Independent counsel Lawrence E. Walsh says he expects to conclude his Iran-contra investigations by the spring of 1991 and to have brought any additional criminal prosecutions by then.

Thereafter, sources said, Walsh will keep his office open only to handle trials of newly indicted individuals or to process possible appeals in cases.

Walsh's investigators are focusing on possible false statements by officials of the CIA, State Department, Pentagon and White House that were given Congress or executive branch inquiries before and after the Iran-contra affair was disclosed in November 1986, the sources said.

In a speech last week in Kansas, Walsh said his investigation had recently obtained additional records pertinent to this inquiry. "He would not have mentioned them if he didn't consider them significant," one source said.

A decision by Attorney General Dick Thornburgh expected in the coming week will directly affect Walsh's investigation into possible false statements by several CIA officials, sources said.

The Justice Department has under review an appeals court decision on Walsh's only case to date against a veteran CIA operative, Joseph F. Fernandez.

Fernandez, the CIA's station chief in Costa Rica from 1984 to 1986, aided the secret contra-resupply effort directed by then-White House aide Oliver L. North, at a time when Congress had prohibited all U.S. military aid to the Nicaraguan rebels.

Fernandez was indicted in April 1989 for making false statements about his role and knowledge of the Iran-contra affair to the White House-appointed Tower Commission and the CIA's inspector general.

Fernandez demanded the use of classified CIA information, arguing he needed the documents for his defense.

But before the start of the Fernandez trial, Thornburgh said in an affidavit filed with the district court in November 1989 that he would not allow use of the material Fernandez demanded because it would cause "serious damage to national security" if it were disclosed.

The issue of possible use of classified CIA information eventually went before the federal court of appeals here, which earlier this month gave Thornburgh until Oct. 6 to make up his mind about whether he will permit the classified CIA material to be used in the trial.

If Thornburgh continues to refuse to allow the classified CIA information to be used in court, Walsh will drop the prosecution of Fernandez on the basis of the appeals court decision, sources said.

"It makes it nearly impossible to prosecute CIA officials as long as they can dredge up classified materials about their work, which the CIA wants to keep secret," one government source familiar with the case said yesterday.

Fernandez's lawyers have said in court papers that they want to use the CIA secret material to show that throughout the time that Congress prohibited any U.S. military aid to the contras, CIA agents "were intimately involved in the resupply of the contras."

During that time and later, CIA officials repeatedly told Congress and the public that they were observing the congressional ban and CIA employees were not involved with the contras.

In the Fernandez case, it has been the CIA, whose officials could be tarnished by release of the materials, that has maintained that the documents must be kept secret.

An interagency security review committee has supported the CIA position which, sources said, was taken to President Bush for final approval.

The recently released 2,600 pages of North's notebooks that covered the 1984-86 period carry many entries about the secret network aiding the contras. It contains frequent mentions of Fernandez and other CIA officials whose activities and testimony, sources said, are being reviewed by Walsh.

These include Clair George, former deputy director for operations, who resigned from the agency in 1987; Alan D. Fiers, former chief of the Central American task force, who left the agency; Duane (Dewey) Clarridge, former head of the European branch and later the agency's terrorism unit, who has left the agency; and James Adkins, a former agency operative who worked directly with the contras in Honduras and was later released by the agency after disclosure of his activities.

Fernandez was accused of falsely telling the Tower Commission that he did not knowingly work with the contra resupply network or know about North's role in the operation.

His defense against these charges, according to court filings, was to attempt to prove that he had no reason to make such false statements because his office in Costa Rica, CIA stations in other countries and CIA headquarters "were all deeply involved in and thus explicitly aware of the resupply operation."

Among the classified CIA documents Fernandez wants for his trial -- and the CIA so far has prevented from being used -- "would show that the private benefactors made several resupply drops of lethal material to the contras from Ilopango airfield" in El Salvador.

North's notebooks have many entries outlining this activity. On Jan. 16, 1986, North records a discussion with Col. James Steele, then the U.S. Army military adviser in El Salvador, which describes the "milestone" to be achieved in setting up shipments from Ilopango. A notation for Jan. 22, 1986, records Steele's report that the El Salvador commander at Ilopango does not want to deal with the local CIA representative on the resupply shipments.

On Jan. 31, 1986, North records that he called Fiers, the CIA's task force chief for the area, and told him that "flight planning data" for Ilopango's resupply flights "should be passed to Steele for hand off to pilots and coord{ination} w{ith} ESAF {the El Salvador Air Force}."