The leftist Sandinista government and the U.S.-backed rebels have broken off peace talks that the insurgents called the final attempt at negotiating an end to the 6-year-old war.

After the negotiations ended Thursday, both sides said a temporary truce would remain in effect for an undetermined amount of time.No new date was announced for resuming the talks, aimed at fashioning a permanent halt to the fighting that has claimed more than 26,000 lives.

Despite the breakdown, rebel leader Alfredo Cesar told journalists Thursday night: "We have reiterated our deep conviction to look for solutions to the conflict in our motherland in a civilized way."

Each side blamed the other for the discussions' failure.

The nation's defense minister, Gen. Humberto Ortega, head of the Sandinista delegation, said the Contra rebels held "irrational positions" and maintained an "anti-negotiation attitude against the peace."

In a statement released in Washington, rebel leader Adolfo Calero countered that the government "demonstrated its hostility toward any meaningful solution to the oppression in Nicaragua."

The goal of the fourth round of negotiations, which began on Tuesday, was to end the war and install democratic reforms, in accordance with a preliminary peace plan signed on March 23 in the village of Sapoa on the Costa Rican border.

Thursday morning, the government put forth its most liberal proposal to date and granted a key rebel demand to institute democratic reforms before the rebels put down their weapons.

The Contras rejected the proposal, saying the Sandinistas did not guarantee that the measures would be enacted before they disarmed.

The Sandinista proposal included freedom of expression, reform of electoral laws, separation of the ruling Sandinista party from the state and the right to strike.

The government proposal also would have granted political amnesty in steps, with the first 200 prisoners freed after the signing of an agreement and the last freed on Sept. 28.

The Contras offered a proposal calling for total amnesty for all political prisoners within five days of signing.

The rebels said there are 4,207 political prisoners, while the International Red Cross, seen here as sympathetic to the government, put the figure at 3,324.

The Contras also demanded separating the armed forces from the ruling Sandinista National Liberation Front in name as well as actions. The government agreed to the "separation and independence of the powers of the state."

The rebels have insisted on a timetable for enacting political reforms, including pluralism, freedom of expression, the right to strike, electoral reforms and guarantees of a mixed economy.

The Sandinista proposal said those changes would be discussed during 60 days among the 14 opposition political parties and eight rebel representatives after rebel fighters gathered in designated zones to disarm.

The talks would be held before the date set for the fighters to lay down their arms. The Contras said they needed guarantees that the democratic measures would not only be discussed but enacted.

The rebels said that if the Sandinistas wanted to show good will, they would bend to their demands.

"It is with deep consternation we announce that an agreement for a permanent cease-fire was not reached," Cesar told reporters.

Ortega, speaking later to reporters, blamed "recalcitrant positions of the (Reagan) administration" and said the rebels had not intended to sign an agreement.

American lawyer Paul Reichler, who serves as a Sandinista negotiator, said earlier that members of the Reagan administration did not want a peace plan to be signed so there would be a chance for additional military aid to the rebels.