MANAGUA, NICARAGUA, NOV. 13 -- While President Daniel Ortega talks peace in Washington, the Sandinista Army is waging a major offensive against U.S.-backed Nicaraguan rebels in the northern, central and southeastern parts of the country.

The offensive began after Ortega announced the suspension, effective Nov. 7, of unilateral cease-fires declared a month before in three zones. In a Nov. 5 speech, Ortega said Sandinista forces would deal with the rebels "with billy clubs and bullets." A fourth cease-fire zone in Nicaragua's sparsely populated Atlantic Coast region was left in place.

Western observers said the Sandinistas seem to be pursuing a strategy of trying to dismantle the rebels, known as contras, through a combination of peace initiatives designed to choke off their U.S. aid lifeline, an amnesty program aimed at whittling down their numbers and battlefield actions to deplete their supplies. Specifically, the offensive is seen as an effort to get the contras to use as much of their ammunition and supplies as possible while U.S. military aid to them is suspended.

In an interview tonight, Vice President Sergio Ramirez explained the offensive by saying, "For us the war has not ended. We're showing our readiness to keep fighting as long as necessary." Ramirez said the contras had taken advantage of the unilateral cease-fire to receive air drops of supplies.

According to the U.S. group Witness for Peace, which opposes contra aid, the rebels have also used the northern truce zone to hold 32 civilian peasants kidnaped during the month-long cease-fire.

Since a $100 million aid package expired Sept. 30, the Reagan administration has received congressional approval for $6.7 million in nonlethal aid for the contras. But it has put off until next year a request for $270 million for the rebels to allow time for implementation of a Central American peace accord.

In line with that accord, Ortega, during a visit to Washington, today initiated indirect truce negotiations with contra leaders for the first time, handing an 11-point proposal to an intermediary, Nicaraguan Cardinal Miguel Obando y Bravo.

Meanwhile, Sandinista-controlled radio stations reported today that government forces are attacking the contras in three military regions that include parts of half the country's 16 provinces.

One progovernment radio report said the Army "broke up" a force of 400 contras in Jinotega province and "annihilated" 17 of them during an operation "initiated due to the suspension of the cease-fire."

The contras' Radio Liberacion reported "major clashes" between rebel and Army units in Jinotega, central Zelaya and Nueva Segovia provinces and charged that the Sandinistas were bombing rural hamlets far from the battle zones.

Some of the fiercest fighting appears to be taking place northeast of the village of Pantasma in an area that was visited from Oct. 31 to Nov. 2 by a group of reporters accompanying a contra unit. It was not immediately clear whether the Sandinista offensive in that area was related to the visit, which produced articles about the contras' support network there.

In one battle Wednesday, contras ambushed a Sandinista military truck on the road north of Pantasma, killing as many as eight Sandinista soldiers, reporters who visited the scene said. They saw two dead contras at the site.

Although the government continues to report battlefield gains and insists that the contras "do not control one single inch of Nicaraguan territory," the Sandinistas are said to be frustrated by their inability to prevent resupply flights to the contras and the failure of an amnesty program to put much of a dent in rebel ranks.

The Sandinistas have acknowledged that the contras receive 25 to 30 air drops a month inside Nicaragua, but no supply plane has been shot down since one that resulted in the capture of American Eugene Hasenfus in October 1986.

The contras have shot down at least 10 Soviet-supplied Sandinista helicopters this year, according to reports confirmed by western observers. The contras have been helped by supplies of U.S. Redeye shoulder-fired antiaircraft missiles, which Ortega denounced in a speech in Washington Wednesday as a threat to civil aviation.

Since the regional peace agreement was signed Aug. 7, the government says, 585 contras out of a total force of 6,500 have taken amnesty under a Sandinista program.

Foreign observers, however, reported that 350 Nicaraguans "at most" have taken amnesty so far, and that some of those are actually Army deserters and draft-dodgers looking for a way out of the war. According to these sources, the contras now have 10,000 to 12,000 fighters inside Nicaragua.