MANAGUA, Nicaragua (AP) _ Cardinal Miguel Obando y Bravo renewed his call Sunday for a Christmas cease-fire in the war between the Sandinista government and U.S.-backed rebels.

''I am troubled by this river of blood,'' the Roman Catholic archbishop of Managua said in his homily at La Sierrita church.

Last week, he served as an intermediary in talks between the government and rebels, known as Contras, held in the Dominican Republic. No progress was reported in the indirect talks, but both sides agreed to meet again.

''Let us make an effort to silence the guns, let us make an effort to negotiate a cease-fire,'' the cardinal said in his sermon. ''There are 50 Nicaraguan youths being killed every day in this war. ... These are not banana trees being cut down ... these are human lives, young people with families, girlfriends.''

In the talks in Santo Domingo, the Dominican Republic capital, Obando y Bravo shuttled between the two delegations because of the government's refusal to meet face-to-face with the rebels, and on his return to Managua the cardinal called for direct talks.

President Daniel Ortega on Saturday rejected that proposal, saying again any direct talks should be with the United States because of its support of the Contras.

He also rejected a rebel offer for a 36-hour cease-fire beginning Monday and Obando y Bravo's proposal that truces be observed Monday and Tuesday for the religious holiday and Dec. 22 through Jan. 7 for Christmas.

Catholics on Tuesdy celebrate the Immaculate Conception of the Virgin Mary. Obando y Bravo urged the Contras and Sandinistas to ''be more flexible'' and said the peace talks should be held ''if not in Nicaragua, at least in Central America.''

''Let us wash our dirty linen at home. ... not go begging in other countries for a place to meet,'' he said.

The talks last week were held in the Dominican Republic, a Caribbean nation, because Ortega said if they were held in a Central American country that would violate the spirit of the regional peace plan adopted Aug. 7.

That plan, signed by the presidents of the five Central American countries at a meeting in Guatemala, calls for cease-fires, amnesties, democratic reforms and an end to foreign aid to insurgents.

On Saturday, the rebels said that if the Sandinistas accepted a 36-hour truce beginning Monday, the Contras would agree to a Christmas cease-fire Dec. 22 through Jan. 6.

But Ortega said the United States must be involved in any truce accoard and the Sandinistas would continue fighting the Contras ''tooth and nail every day of December.''

''Our position is that we cannot give a truce to the Contras, to those mercenaries, those terrorists who are killing our people,'' he said.

In San Jose, Costa Rica, an assembly of Contra representatives opened a three-day meeting to discuss a proposal for forming a national unity government in Nicaragua.

The assembly of the Nicaraguan Resistance, the rebels' umbrella organization, also is expected to discuss the regional peace plan and proposals for a new constitution that would be put into effect under a different government in Managua.

The assembly has 54 members from various sectors opposed to the Sandinistas.