Talks on ending the leadership crisis in Honduras broke off Sunday after the interim government declared "unacceptable" a proposed compromise that included the return of ousted President Manuel Zelaya to serve out his term.

The two camps' positions remain far apart after a fourth day of negotiations, but the mediator, Costa Rican President Oscar Arias, promised renewed efforts to seek a solution and avoid bloodshed in the Central American country.

"It was not possible to reach a satisfactory agreement. The Zelaya delegation fully accepted my proposal, but not that of Don Roberto Micheletti," Arias said, referring to the interim president sworn in by congress after the June 28 coup.

Arias aims to renew talks in 72 hours

Arias said he will spend the next three days "working much harder to see if we can reach an agreement, because what is the alternative to dialogue?"

On Saturday, Arias proposed a plan that would let Zelaya serve out the final months of his term, move up elections by one month to late October, grant a general amnesty and include representatives of the main political parties in a reconciliation government.

Arias, the 1987 Nobel Peace Prize laureate for brokering an end to Central America's civil wars, had urged patience from Zelaya and flexibility from the interim government, which has ruled since the military whisked Zelaya out of the country at gunpoint.

The Micheletti government endorsed several of Arias' proposals on Sunday, but Foreign Relations Secretary Carlos Lopez rejected the overall plan, specifically citing its provision for Zelaya's return.

"Dear mediator … I'm very sorry, but your proposals are unacceptable," Lopez said at a news conference after the talks. Arias' compromise, he added, "interferes with Honduran internal affairs."

Minutes later, Lopez told CNN en Espanol that his delegation would return to the Costa Rican capital on Wednesday "to continue our conversations."

But Enrique Flores, a negotiator for Zelaya, said that while Arias may continue "personal efforts" to reach an agreement, formal talks are over.

"Today, the dialogue ended," Flores said.

Instead, the interim government offered to create a truth commission to "let the Honduran people and the international community see all the acts that led to the current situation," according to letter signed by Lopez.

Zelaya plans to go back

Zelaya, who previously vowed to return to Honduras to set up a parallel government if the talks failed, told The Associated Press that he was willing to leave "the door open for diplomacy and dialogue."

His aide, Allan Fajardo, has said that Zelaya planned to go back before Friday, the date suggested by Arias for his return.

"The president is preparing his return to Honduras, with or without an agreement," Fajardo said.

The Honduran military thwarted Zelaya's first attempt to fly home on July 5 by blocking the runway at the airport in the capital, Tegucigalpa.

Zelaya's ouster is a major test of Latin American democracy and of the Obama administration's policy toward the region. The United States, the United Nations and the Organization of American States have demanded that Zelaya be returned to power, and no foreign government has recognized Micheletti.

Honduran labour groups supporting Zelaya called for a general strike Thursday and Friday.

And in Nicaragua, Zelaya's Foreign Minister Patricia Rodas called for a massive march if mediation fails. She attended the 30th anniversary celebrations of the Sandinista revolution there on Sunday.

Honduras' Supreme Court issued an arrest warrant for Zelaya before the coup, ruling that his effort to hold a referendum on calling for a constitutional assembly was illegal.

Many Hondurans viewed the referendum as an attempt by Zelaya to push for a socialist-leaning government similar to the one his ally Hugo Chavez has established in Venezuela.