MANAGUA, NICARAGUA, SEPT. 30 -- A month-long, unilateral government cease-fire in the war against the U.S.-backed rebels will begin Oct. 7 in three of Nicaragua's most embattled provinces, President Daniel Ortega said tonight.

Nicaragua will withdraw its regular troops from three cease-fire zones in advance of the Nov. 7 deadline of a regional peace plan to "prevent bloodshed" and move gradually toward the nationwide cease-fire required by the accords, Ortega said.

This would be the first formal hiatus in the hostilities since rebel guerrillas, known as contras, began fighting the government in 1981.

The zones are in Nueva Segovia and Jinotega provinces in the north and Zelaya province in the south, covering a total area of 550 square miles. Army troops operating there are to be pulled back starting Oct. 7 to towns on the edge of the zones.

However, Ortega said Sandinista peasant militias living on farming cooperatives inside the zones will remain armed and in their homes. If the militias are attacked, Sandinista regulars will enter the zones to pursue the attacking contras, he said.

The government expects many contras to lay down their arms and return to civilian life under an amnesty included in the peace plan. The first three cease-fire zones are in effect until Nov. 7, but the government can prolong them and open new zones, according to a communique released tonight.

While no reaction was immediately available from the contras, in recent days they have given mixed signals whether they will stop shooting if they are not included in negotiations leading to the cease-fire.