Salvadoran President Jose Napoleon Duarte announced today that the military believes guerrilla forces will begin an anticipated nationwide offensive here Friday.

Duarte also confirmed in a television broadcast a number of changes within the civilian-military coalition government that left most Cabinet ministers in place, replaced some officials accused of extreme rightist ties and brought back at least one member of the military leadership that has overthrown in a coup by so-called "progressives" 15 months ago.

The changes follow insistence by the Christian Democratic Party, which Duarte represents in the coalition, as well as the United States and other Western governments here that officials allegedly tied to right-wing paramilitary forces and opposed to reforms be weeded out of the government.

The insistence came on the heels of a suspension of U.S. military aid following the murder last month of four American churchwomen and what U.S. officials said was at least circustantial evidence of government troop involvement.

The promised changes began in December, however, with the ouster from the then-ruling junta of its most progressive member, Col. Aldolfo A. Majano, leader of the Army's reformist military youth movement.

The Christian Democrats -- decimated by the regisnations of officials to protest alleged repression of peasants by security forces -- had demanded other changes as a condition for their remaining in the coalition.

One of those was the naming of Duarte as civilian president to replace the junta as the national executive. Duarte was sworn in several weeks ago. Another initial demand reportedly was the removal of Defense Minister Guillermo Garcia. But according to Duarte's announcement today, Garcia remains in his job.

Deputy Defense Minister Nicolas Carranza, who along with Garcia had been accused of tolerating abuses by government troops, has been transferred to head the government telecommunications department.

Carranza's replacement is the former armed forces chief of staff, Col. Francisco A. Castillo. To the surprise of many observers, however, the new staff chief is Army Col. Rafael Flores Lima, who served as press secretary to former president Carlos Humberto Romero -- the Army general who was overthrown in 1979. Like many Army officers considered to be ultraconservatives or tightly bound to Romero, Flores Lima had been transferred to a diplomatic post outside the country at that time.

Introduced by Garcia at a news conference yesterday, Flores Lima, who apparently was called back as soon as Majano was thrown out of the junta, said that the "armed forces have been too patient" in combating the left during his absence.

In another change, Christian Democratic junta member Antonio Morales Erlich has been named to direct the agrarian reform program that the government and its U.S. backers hope will offset the attraction of the left here by more equitably distributing productive land. The program has been violently opposed by both the right and left.

There was no announced replacement, however, for agrarian reform institute head Rodolfo Viera, who was killed along with two American advisers last Saturday night by unidentified gunmen.

Overall, the changes appear to strengthen the rightists in the government and to fall short of what the Christian Democrats had demanded from the conservative military leaders with whom they ostensibly share power.

One Western diplomat said there is an increasing "shortage of whole people" -- those who have not either been killed or left the country in fear or joined the opposition -- who are willing to take government jobs.

Washington Post Correspondent Christopher Dickey added from Mexico City:

Duarte's announcement that a guerrilla offensive is scheduled to begin Friday apparently results from government interception of a secret guerilla communication.

The communication, outlining plans for the offensive, reportedly came from the Popular Liberation Front, the most powerful of half a dozen allied guerrilla groups. Although there have been reports that the offensive was about to begin in the past, the government reportedly is taking the call to arms more seriously this time since the front has been more reluctant than its colleagues in the past to call for an all-out war.

The front's strategy has been clearly articulated for some time, including the establishment of "liberated zones" for training camps and supply-route consolidation, before mounting an offensive. Although no such zones exist and the guerillas appear to control only small villages in remote areas, the front is believed to have acquiesced to the offensive plan because of a desire among all the guerilla groups to create what one leader last week called an "irreversible situation" before President-elect Ronald Reagan is inaugurated Jan. 20.

The intercepted communication, sources here said, calls for attacks on banks, schools and factories to shut the country down. Western military analysts believe the goal is to leave workers and students free to man the barricades under the banner of a "final offensive."

In his speech, Duarte said that the country's military is "ready" to combat the offensive, which he said had no popular support.

In Santa Ana, El Salvador's second-largest city, United Press International reported, police circulated leaflets telling citizens to raise their hands if combat erupted, to show security forces they were unarmed.

[A Salvadoran Army brigade commander, meanwhile, said troops killed 50 guerrillas outside a town 50 miles west of the capital. However, the nation's human rights commission launched an investigation because of reports that some of those killed were innocent civilians.]