GUATEMALA CITY--Peace talks between the leftist Nicaraguan government and contra rebels broke down yesterday, with their mediator indirectly blaming the Sandinistas for not accepting a new cease-fire proposal.

Miguel Cardinal Obando y Bravo, the head of the Roman Catholic Church in Nicaragua, suspended the talks in midafternoon, expressing disappointment that his proposal had not been accepted "in principle" by both sides.

Victor Hugo Tinoco, Nicaragua's deputy foreign minister, said the cardinal called off the talks before he could deliver a formal, written response that would have accepted the mediator's five-point plan.

The plan, offered by Obando y Bravo on Thursday, called on the contras to accept a cease-fire in exchange for four major concessions by the Sandinistas, including a total amnesty for all political prisoners and full freedom of the press.

The cardinal's communique said that "after all the meetings that they have had, the mediators believe that the acceptance, in principle, of its proposal would favor the rapid advance of the cease-fire negotiations."

The communique also said the government and the contras should give their negotiators "enough decision-making ability to resolve problems that present themselves in the negotiations."

In a news conference after the talks ended, Jaime Morales, the chief contra negotiator, said his team had the ability to make decisions on the spot, noting that the rebels had accepted Obando y Bravo's proposal Thursday night.

"We took a position and the press is witness to the fact," Morales said. "We accepted the proposal in principle, and the other side did not."

Tinoco, on the other hand, blamed the contras for the breakdown in the negotiations.

"It was our impression that the contras were systematically rejecting all discussion of a cease-fire," he told reporters. "The contras continue to be advised by people in the [Reagan] Administration who want to continue the war."

An attorney quoted the Nicaraguan president as saying he would be willing to talk to Obando y Bravo about his proposal but did not want his negotiators to discuss it at the contra talks.

Later in the day, however, the government apparently switched positions. But by the time it could get its new response to the cardinal, he had called off the negotiations.

Tinoco said the government's formal response accepted the cardinal's five-point proposal in principle, but said it was "indispensable to arrive at a definition and agreement" on his cease-fire plan.