Updated

Negotiations to end Honduras' political crisis are facing a new challenge after the ousted president vowed to act on his own if he is not returned to power in the next round of talks, possibly this weekend.

Manuel Zelaya, who was toppled by a military-backed coup and flown out of the country in his night clothes on June 28, is clearly frustrated by the slow movement of negotiations mediated by Costa Rican President Oscar Arias, which have produced no breakthrough after two rounds.

"We are giving the coup regime an ultimatum," Zelaya said Monday at a news conference in Nicaragua, where he arrived Sunday night following a brief trip to Washington.

If at the next round of talks the interim government does not agree to reinstate him, "the mediation effort will be considered failed and other measures will be taken," he said. He did not say what those measures would be.

The interim government of Roberto Micheletti, which insists the coup is legal since it was backed by Honduras' Supreme Court and Congress, has refused to bend on reinstating Zelaya and is trying to make life return to normal in the impoverished Central American nation.

On Sunday, the government lifted a nighttime curfew in place since the coup and it successfully urged tens of thousands of Honduran teachers and students to return to class Monday.

Micheletti said late Monday that a Honduran negotiating team could return to the bargaining table as early as this weekend to try to end the stalemate caused by the coup.

At the swearing-in ceremony of a new foreign minister Monday, Micheletti said his team of delegates was "ready for another meeting."

Zelaya accused the Micheletti government of using the talks "as a means to distract attention" from repression in Honduras, where protests for and against Zelaya's return have filled the streets, though they have waned in recent days.

Members of Micheletti's administration did not immediately respond to Zelaya's comments.

Arias, the 1987 Nobel Peace Prize winner for his role in helping end Central America's civil wars, was expected to announce the date for new talks soon.

In Washington, State Department spokesman Ian Kelly reiterated U.S. support for Arias' mediation efforts.

"It is not a process that's being led by the United States of America. We just have to give time for this process to work. And I'll just say, we're standing firmly behind President Arias," Kelly said.

Despite Kelly's comments, Washington has clearly been playing an influential role in the negotiations: It was U.S. Secretary of State Hillary Clinton who invited Arias to mediate and Zelaya supporters have been urging the United States in particular to take firm action that they say would force the interim government to back down.

The coup has drawn international condemnation and nations have urged that Zelaya be restored to his post as the democratically elected president.

Honduras' Supreme Court, Congress and military say they legally removed Zelaya for violating the constitution. They accuse him of trying to extend his time in office. But Zelaya denies that, saying he is the democratically elected leader of Honduras who was toppled by the country's military and business elite.

Both Zelaya and Micheletti, the congressional president who was appointed by lawmakers to serve out the final six months of Zelaya's presidential term, met separately with Arias last week but they refused to talk face to face. Zelaya is scheduled to travel to Guatemala on Tuesday, where he will meet with President Alvaro Colom, said Ronaldo Robles, a spokesman from Colom's office.

Former Honduran Foreign Minister Carlos Lopez, a Micheletti representative at the talks, said his side had not ruled out the possibility of early elections as a way out of the crisis.

As the crisis drags on, the interim government has urged people to resume their lives as usual in Honduras. About 38,000 teachers heeded a request to return to class Monday — but more than 20,000 remained on strike to protest the coup.

High school director Alejandro Ventura said that thousands of teachers decided to go back to school because there is no solution in sight for the political crisis. "We cannot keep acting irresponsibly with our children," he said.

Most of the children affected by the walkout also returned to class, said teacher's union leader Eulogio Chavez.

Zelaya supporters have said the interim government is trying to restore normality to the nation with the hope of sapping the energy of the protest movement and staying in power until the November presidential election.

Daily demonstrations for and against Zelaya continue in the country, but are steadily losing steam. Turnout at demonstrations has fallen from several thousand to only a few hundred this week.

"Each day fewer people come and our leaders are disorganized," said Rosaura Izaguirre, one of about 300 Zelaya supporters blocking a main road connecting the Caribbean coast to the capital for two hours Monday.