SAN SALVADOR, AUG. 14 -- El Salvador's leftist rebels today accepted an offer by President Jose Napolean Duarte for direct peace talks, but Duarte later said his government could not meet with them under the conditions they proposed.

Despite rejecting the rebels' acceptance of his offer, Duarte said the rebels' communique today was a step forward. "It's a good start that they are saying, 'We are ready to talk,' but it is not sufficient," he said in an interview with local journalists.

In a nationally televised speech last night, Duarte proposed talks with the guerrillas in September, taking a step toward implementation of a peace proposal signed last week by all five Central American presidents.

The coalition of Salvadoran rebels, known as the Farabundo Marti National Liberation Front, and their political wing, the Democratic Revolutionary Front, announced earlier today that they would accept Duarte's offer, suggesting that two-day talks be held at the office of the Vatican envoy here. The communique, read on Salvadoran radio, also suggested that Archbishop Arturo Rivera y Damas act as mediator.

The rebel statement, however, did not mention any intention of renouncing their armed struggle and fell short of a full acceptance of the Central American peace accord.

Duarte said today that the leftist rebels ignored key government stipulations in accepting his proposal.

He said that in order for the government to accept a meeting with the rebels, the guerrillas would have to publicly renounce violence as a means of gaining power.

"The day that they say that violence is not an instrument for taking power and that they want to incorporate themselves into the democratic process, I will immediately meet with them," Duarte said.

However, he said he was not insisting that the rebels lay down their arms as a precondition to talks.

Duarte reiterated his stance, outlined in his televised address last night to the Legislative Assembly, that the rebels must publicly accept by Aug. 30 a regional peace plan signed last week in Guatemala City by Duarte and the four other Central American presidents.

The peace plan called for cease-fires between governments and rebel groups in the region within 90 days, an end to foreign support to insurgent groups and "democratization" of all governments.

If held, the talks would be the first time since 1984 that the government and rebels have held negotiations in an effort to end the civil war that began in 1979. Duarte and senior military officials here have said in the past that they would not agree to talks with the rebels until they abandoned their armed struggle.

{Francisco Altschul, a representative of the Revolutionary Democratic Front in Washington, said, "The spirit of the {Guatemala} accord is positive," adding that "within that framework" the Salvadoran rebels wanted to resume talks with the government "without previous conditions," Terri Shaw of The Washington Post Foreign Service reported.}

{Meanwhile, in Guatemala, where an insurgency is challenging Christian Democratic President Vinicio Cerezo, the guerrilla coalition announced that it would be willing to talk with the government but would not lay down arms before the talks were held.

{The peace plan's call for democratization has led to pressures on the Sandinista government in Nicaragua to reopen the opposition newspaper La Prensa, closed June 26, 1986, after publishing for years under heavy censorship. Earlier this week, Costa Rican President Oscar Arias, author of the peace plan, asked the Sandinistas to restore freedom of the press. On Friday, U.S. Senate Majority Leader Robert C. Byrd (D-W.Va.) sent Ortega a letter urging him to allow La Prensa "to resume uncensored publication at the earliest possible date."}

Meanwhile, Honduran President Jose Azcona said today that if Nicaragua does not comply with the peace plan, U.S. allies in the region will not ask the United States to suspend aid to Nicaraguan rebels, or contras.

"If Nicaragua does not comply with the accord, then neither will the United States receive a request that the aid to the contras be suspended," Azcona said in Tegucigalpa, the Honduran capital.

Although the White House has expressed cautious support for the peace plan, President Reagan has said he intends to protect U.S. interests and those of the contras fighting from bases in Honduras to overthrow Nicaragua's leftist Sandinista government.

Azcona said the decision to continue supporting the contras is the correct one from the U.S. point of view because it is not yet clear if the parties are going to comply with the accord.

However, he said his government intends to comply with the accord by not allowing the contras to use bases in Honduras.