SANTO DOMINGO, DOMINICAN REPUBLIC, DEC. 4 -- The first round of talks aimed at ending the Nicaraguan war reached an impasse today after the Sandinista government rejected the mediator's proposal for a Christmas truce.

Cardinal Miguel Obando y Bravo, the Nicaraguan mediator, said government negotiators had sought to modify his proposal with conditions that were unacceptable to the U.S.-backed rebels, known as contras.

"It seems we have reached a deadlock," the cardinal told reporters after two days of separate meetings with both sides.

In a lengthy report on the 5 1/2 hours of talks, Obando said the rebels had accepted his proposal, and he blamed the Sandinista government for the lack of an agreement. He said he would urge Nicaraguan President Daniel Ortega to accept face-to-face talks between the two parties to speed up peace efforts.

Nicaragua's chief negotiator, Deputy Foreign Minister Victor Hugo Tinoco, later rejected direct talks, leaving the next step unclear. Obando said he would "reflect" on whether to continue his mediation effort and, if he does, set a date for new meetings. The government negotiators proposed a second round of indirect talks Dec. 17 and 18.

Bosco Matamoros, a spokesman for the rebel delegation, said, "We are ready to meet again whenever the cardinal chooses."

The talks stem from an Aug. 7 peace accord signed by five Central American presidents. The agreement says cease-fires in the region's guerrilla wars are to take effect simultaneously with other steps: democratic reforms, amnesty for rebels and a cutoff of their outside aid and sanctuaries.

The impasse here seems to result from the difficulty of orchestrating all those steps at once in Nicaragua.

Obando proposed that the government act first. As conditions for a truce, he asked that the Sandinistas allow unrestricted press freedom, grant amnesty to all political prisoners and lift a wartime state of emergency that suspends most civil liberties.

He called for a 36-hour truce for the Roman Catholic Feast of the Immaculate Conception on Dec. 7 and 8 and another truce from Dec. 22 through Jan. 6, the Feast of the Epiphany.

In accepting the proposal, the rebels agreed to postpone their demands for far-reaching political concessions and not to make them a condition for halting the fighting.

But the Sandinistas, citing the regional peace accord, asked Obando to add two conditions to the truce -- that the contras stop receiving aid from the United States and using bases in neighboring Honduras.

Obando said the rebels were willing to do that only if the Sandinistas acted first to "begin an irreversible process of democratization" in Nicaragua.