President Reagan said last night that a CIA employe in Central America had removed several pages from a guerrilla-warfare manual after recognizing that it violated the president's order barring any U.S. role in political assassinations, but "some way or other there were 12 of the original copies that got out down there."

Reagan said that an investigation was continuing, and White House national security affairs adviser Robert C. McFarlane said yesterday that any Reagan administration official found to be involved with the Central Intelligence Agency primer instructing the antigovernment "contras" in Nicaragua on assassinations, blackmail and mob violence would be fired.

During his foreign-policy debate last night with Walter F. Mondale, Reagan said the manual was written by "a gentleman down in Nicaragua who is on contract to the CIA, advising -- supposedly on military tactics -- the contras."

Reagan said the administration would "take the proper action at the proper time" after determining "where any blame lies for the few that did not get excised."

Mondale responded, "This manual, several thousand of which were produced, was distributed, ordering political assassinations, hiring of criminals and other forms of terrorism. Some of it was excised, but the part dealing with political terrorism was continued."

According to Reagan, the manual was delivered to a CIA official in Central America, who "recognized that what was in that manual was a direct contravention of my own executive order in December of 1981, that we would have nothing to do with regard to political assassinations." He said the CIA official excised several pages before passing the manual on to the CIA, where more pages were deleted.

McFarlane, on "This Week With David Brinkley" (ABC, WJLA), said that no Reagan administration official had been linked to the 90-page manual. He said the evidence suggested it was solely the work of Nicaraguan rebels and the CIA contract employe without the knowledge of U.S. officials.

"I think the president has made clear that if there were U.S. officials involved in the development of this and approval of it, they ought to be fired," McFarlane said.

Later he said, "All of the evidence preliminarily is that this was a subordinate official and perhaps not even someone who was employed by the CIA."

Asked if CIA Director William J. Casey would be fired if found to have been involved, McFarlane said: "Whoever was involved ought to be fired." He said that an investigation should be completed within a week.

Former president Jimmy Carter yesterday joined Democrats calling for Casey's resignation because of the CIA manual. Mondale, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), Rep. James M. Shannon (D-Mass.) and Rep. George Miller (D-Calif.) have demanded Casey's ouster.

McFarlane's latest statements about the CIA manual came amid reports suggesting that the Nicaraguan contras, whom Reagan calls "freedom fighters," were executing officials of the Sandinista government as early as 1982, and that senior Reagan aides had discussed the need for a guerrilla-warfare handbook last year.

Newsweek magazine reported yesterday that aides to U.N. Ambassador Jeane J. Kirkpatrick, Casey and Defense Department officials discussed the need for such a handbook in August 1983.

Meanwhile, in Miami, a Nicaraguan rebel leader told the Associated Press that his group had a "practice" of executing officials of Nicaragua's government, mostly security officials in small towns. "We have taken towns, and our men have had to kill officials of the Sandinista government," said Edgar Chamorro, chief of propaganda for the Nicaraguan Democratic Force.

"In guerrilla war, if you have to exact justice immediately, sometimes you have to do it," he said. "We don't have jails. We are in the jungle." The Reagan administration has denied seeking to overthrow the Nicaraguan government, which came to power in the rebellion against Anastasio Somoza in 1979. But Reagan has advocated support for the contras by arguing that the Sandinistas are supplying arms to leftist guerrillas in El Salvador.

Secretary of State George P. Shultz asserted again yesterday, "There isn't any doubt whatsoever that the Salvadoran guerrillas are supplied by Nicaragua."

Speaking on "Meet the Press" (NBC, WRC), Shultz said that evidence of Nicaraguan arms supplies to Salvadoran rebels was being obtained by CIA surveillance flights that became public Friday, when four CIA employes were killed in the crash of their small unarmed plane three miles from San Salvador, the country's capital.

Shultz said that because "the situation in El Salvador has been gradually improving" U.S. officials there had begun traveling more freely about the country, venturing farther and farther from the capital.