MEXICO CITY, DEC. 7 -- The Nicaraguan rebels said today they would observe a 36-hour truce in their war against the Sandinista government, heeding a call by Nicaraguan Cardinal Miguel Obando y Bravo.

In a statement issued in Miami, the Nicaraguan Resistance, the political alliance of the rebels, known as contras, said that as a "show of good faith" in cease-fire negotiations being mediated by Obando, contra forces in Nicaragua would observe a truce from noon today to midnight Tuesday to mark the Day of the Immaculate Conception, a Roman Catholic holy day.

"Our forces have orders to fire only if they are attacked," said Pedro Joaquin Chamorro, one of six civilian directors of the Nicaraguan Resistance, who was interviewed by telephone from Miami.

Obando proposed the 36-hour truce, plus a cease-fire from Dec. 22 to Jan. 6 for the Christmas and New Year holidays, during indirect negotiations last week between the contras and the Sandinistas in Santo Domingo, the capital of the Dominican Republic.

In its statement today, the Nicaraguan Resistance said it would announce its position soon on the Dec. 22-Jan. 6 truce suggested by Obando, "after observing Sandinista compliance" with the 36-hour truce. The statement reiterated the contras' call for direct talks with the Sandinistas.

The Sandinista government has already rejected the truces as well as a call by Obando for face-to-face negotiations with the contras.

"If a cease-fire is desired, the United States must cease fire because they are the ones who are making war on Nicaragua," Nicaraguan President Daniel Ortega said in Managua Saturday. "Our position is that we cannot give a truce to the contras, to those mercenaries, those terrorists who are murdering our people. We are going to continue hitting them hard during this whole month of December."

Nicaraguan Defense Ministry officials said the government would not accept a holiday truce as long as U.S.-financed supply flights to the contras from Honduras continued.

Costa Rican President Oscar Arias, en route to Oslo to accept the Nobel Peace Prize, urged that cease-fires in Central America's wars be reached by Christmas, saying that intransigence and dogmatism were the main obstacles to peace in the area, news services reported.

Arias won the peace prize for drafting a Central American peace plan signed Aug. 7 by the presidents of Nicaragua, El Salvador, Guatemala, Honduras and Costa Rica.

Arias said at a news conference that he would be willing to visit Havana for talks with Cuban leader Fidel Castro to gain his support for the peace plan because without his help, "it is going to be very difficult to comply with the Guatemala accords."

It was to show compliance with Arias' plan that the Sandinistas reversed a longstanding position last month and agreed to indirect cease-fire talks with the contras, which ended in deadlock last week.

On Nov. 13, Ortega proposed a 30-day truce starting Dec. 5 during which the contras would be restricted to three cease-fire zones, totaling 4,169 square miles, in remote areas of Nicaragua. At the end of the cease-fire, the Sandinistas proposed, the contras would turn in their weapons, accept a government amnesty and join the political process.

The contras rejected this as a proposal for their "orderly surrender."