An assistant federal prosecutor in Miami said yesterday that six months before the Iran-contra scandal erupted he and the FBI had urged that a grand jury investigate possible illegal gun-running to the Nicaraguan contras.

Jeffrey Feldman, the assistant U.S. attorney in charge of the controversial Miami inquiry, said that although "there was a delay" in launching the investigation, he has no evidence that his boss, Leon B. Kellner, the U.S. attorney for Miami, acted improperly or held up the grand jury investigation to prevent embarrassment to the Reagan administration's program supporting the anti-Sandinista rebels.

A House Judiciary subcommittee has been investigating whether the Justice Department and Kellner attempted to slow the gun-running probe to prevent discovery of crimes that might have undermined congressional support for the rebels. The subcommittee, chaired by Rep. William J. Hughes (D-N.J.), has taken sworn testimony from Feldman, Kellner and other officials in the Miami U.S. attorney's office, but has not issued a report on its findings.

Kellner, who could not be reached for comment yesterday, has said he was in regular contact with senior Justice Department officials about the inquiry, but they made no attempt to pressure him. He said he regularly briefs top officials.

Previous testimony before the congressional Iran-contra panels has revealed that early last year associates of fired National Security Council aide Lt. Col. Oliver L. North were concerned that the Miami investigation might expose his secret contra support operation.

Kellner, in an interview earlier this year, said he did not approve Feldman's original request because he and other senior aides wanted more evidence before deciding whether a grand jury investigation was justified. The grand jury investigation was approved by Kellner in November 1986 after Feldman and the FBI developed additional information.

The investigation, which is still pending, is examining allegations of weapons shipments from Florida to the contras in Central America in 1985.

Ana Barnett, an aide to Kellner, said yesterday that "we have nothing to hide," and urged Hughes' subcommittee to release all the testimony it has taken on the inquiry.

She and other Kellner aides said they were disturbed by a story in yesterday's Miami Herald disclosing parts of Feldman's testimony to Hughes's panel. The Herald story also quoted Feldman.

In a telephone interview with The Washington Post yesterday, Feldman said he has learned that, without his knowledge, part of a memo he gave Kellner in late May 1986 outlining the investigation was revised by another Kellner aide. Feldman said the changes contained language that strengthened the argument for holding off on the grand jury investigation.

Feldman said that one of the added sentences read: "Absent further field investigation, a grand jury investigation at this point would represent a fishing expedition with little prospect that it would bear fruit." Feldman said the revised memo was then sent to the Justice Department in Washington.

Lawrence H. Sharf, a senior Kellner aide, said yesterday that he wrote the changes in the memo after Kellner asked him to review what Feldman had written. Sharf said he had attended the meeting where Kellner, Feldman and other Kellner aides had discussed what should be done.

Sharf said he did not know at the time that Feldman was not shown the revisions. "I've been in the Justice Department for 10 years," Sharf said. "I made certain calls in my judgment that were based on what I thought was for a sound investigation and prosecution strategy."

Feldman said that he does not know if there were any ulterior motives behind Kellner's decision on the grand jury. "I can't tell you if {Kellner's} actions were bad or good," Feldman said. "There was a period of delay. That's all I know."