MIAMI (AP) _ Nicaraguan rebel leaders Tuesday accused their country's leftist government of unleashing a pre-election campaign of repression and murder, and a human rights group offered some unprecedented confirmation.

''The Sandinista government is massacring supporters of the resistance,'' Contra director Aristides Sanchez said at a news conference in the group's Miami headquarters. ''They are wiping out our entire social base.''

The Contras accused the Sandinista military of slaughtering 40 to 50 civilians over the past two months in northern Nicaraguan provinces where the U.S.-supported rebels were once strongest, including 13 in the town of Apantillo.

For the first time, the widely respected human rights group Americas Watch, which in the past has criticized the Contras, agreed that the Sandinistas are killing suspected rebel sympathizers in the countryside.

''We cannot dismiss them as isolated incidents any more,'' said Americas Watch spokesman Juan Mendez in Washington. ''We have written the Nicaraguan government and are waiting to see what they are doing about this.''

The withdrawal of armed Contras that followed a recent series of peace agreements has left their supporters open to repression, said Adolfo Calero, another of the seven rebel directors.

''They are taking advantage of the truce and the peace agreements to eliminate all our supporters,'' Calero said.

A physician, who calls himself ''Dr. Henry'' and who has treated Contra troops and sympathizers inside Nicaragua for five years, said the Sandinistas also have mined roads along the Honduran and Costa Rican borders, resulting in the deaths of eight innocent victims.

On another matter, Calero charged that the Supreme Electoral Council, newly created to oversee next year's planned national elections, violates promises the Sandinistas made at a meeting of Central American presidents in El Salvador to create a ''balanced'' commission.

He said President Daniel Ortega and the Sandinista-controlled national assembly will appoint and approve all five members, including two representative s of the opposition.

''This is no way to initiate the electoral process in Nicaragua,'' Calero said. ''And we expect the Central American presidents to demand it be changed.''

The Contras, who have just won a $50 million non-lethal aid package from the United States, denied they have been pressured by the Bush administration to return to Nicaragua to participate in the efforts to democratize the country.

''I have heard nothing from the Bush administration, and I will return to Nicaragua when I decide to, which is when the internal opposition asks us to return,'' Calero said.