The Reagan administration engaged in illegal "covert propaganda activities" designed to influence the news media and the public to support its Central American policies, according to a report by the congressional General Accounting Office released yesterday.

The report said the State Department's Office of Public Diplomacy for Latin America and the Caribbean had violated a congressional ban on the use of taxpayers' money for unauthorized publicity and propaganda purposes in 1985.

In a statement releasing the report, Rep. Jack Brooks (D-Tex.), chairman of the House Government Operations Committee, said that "this illegal operation represented an important cog in the administration's effort to manipulate public opinion and congressional action."

Rep. Dante B. Fascell (D-Fla.), chairman of the House Foreign Affairs Committee, said, "It makes me wonder what else is still being hidden from Congress and the American people."

Attached to the GAO report is a "confidential eyes only" memorandum of March 13, 1985, to Patrick J. Buchanan, then White House communications director, from Johnathan S. Miller, then an official of the State Department Latin America public diplomacy unit, boasting of its "white propaganda" operations. Later Miller became a White House administrative aide and resigned last May after it was revealed that he had cashed traveler's checks from Lt. Col. Oliver L. North's National Security Council safe for payment to a contra leader.

In a related development, President Reagan prepared to inaugurate a new phase of his campaign to continue U.S. aid to the Nicaraguan contras in a speech Wednesday, demanding additional actions by Nicaragua to change its internal policies and meet U.S. national security concerns, White House sources said yesterday.

Reagan's speech to the Organization of American States, followed by an address by Secretary of State George P. Shultz in Chicago Friday, was described as the kickoff of a month-long drive that will culminate in a request to Congress shortly after Nov. 7 for additional contra aid.

The administration's efforts to continue contra aid have been complicated by Nicaraguan government acceptance of a Central American peace agreement calling for major policy shifts, including greater political freedoms and other reforms, by Nov. 7.

The New York Times reported yesterday that the White House has settled on a list of demands to be made of the Nicaraguan government that go beyond the Central American peace agreement. The Times said these demands include new presidential elections, an end to Soviet and Cuban military aid, negotiation of a Nicaraguan cease-fire directly or indirectly with the contras, full amnesty for the contras, release of all Nicaraguan political prisoners and sharp reduction in the size of the Nicaraguan armed forces.

A White House official confirmed that these points have been under discussion in the executive branch, but said it is uncertain whether Reagan will spell them out in his OAS address Wednesday.

Nicaragua's ambassador to the United States, Carlos Tunnermann, said yesterday that "it is not up to the government of the United States to take upon itself the right to tell the presidents of the Central American countries to add to the Guatemala accords." Tunnermann said Nicaragua would refuse to change the timing of its next presidential elections, which are scheduled for 1990 under the country's constitution.

In the "white propaganda" memo to Buchanan released yesterday in the GAO report, Miller cited the Wall Street Journal's publication on its March 11, 1985, op-ed page of an article by John F. Guilmartin Jr. on Soviet arms aid to Nicaragua.

Guilmartin was identified by The Journal only as a professor at Rice University and a former U.S. Air Force officer and editor of the Air University Review. However, Miller told Buchanan in the memo that "Professor Guilmartin has been a consultant to our office and collaborated with our staff in the writing of this piece . . . . Officially, this office had no role in its preparation."

Guilmartin, now an associate professor at Ohio State, said yesterday that he had received "less than $1,000" as a State Department consultant on Nicaraguan arms but that his Wall Street Journal article was "mine and only mine" without any collaboration with the public diplomacy office.

Robert L. Bartley, Wall Street Journal editor, said he would look into the circumstances of the Guilmartin article before making further comment.

The "white propaganda" memo also said that op-ed pieces for The Washington Post and The New York Times were "being prepared by one of our consultants" for the signatures of then-contra leaders Alfonso Robelo, Adolfo Calero and Arturo Cruz.

The Post carried an op-ed article by Calero on April 7, 1985, but deputy editorial page editor Stephen S. Rosenfeld said yesterday he worked directly with Calero on the article and had no indication of a State Department hand in it. The Times carried an op-ed article by the three contra leaders on Dec. 13, 1985. Robert B. Semple, editor of The Times op-ed page, said it was "far from clear" that the article was the same one "that apparently was being prepared the previous March."

The GAO report said the unit's activities violated a law against the use of State Department funds "for publicity or propaganda purposes not authorized by Congress." Otto Reich, who was head of the Latin American public diplomacy office at the time and is now U.S. ambassador to Venezuela, said he had not been interviewed by the GAO, and that his office "did not engage in any kind of propaganda of any color." Staff writer Terri Shaw contributed to this report.