El Salvador's left-wing rebel alliance today agreed conditionally to meet with President Jose Napoleon Duarte on Monday in what would be the first high-level peace talks in five years of civil war.

The Salvadoran government did not formally comment on the points listed in a rebel communique as conditions for the insurgents' participation. These included a demand that senior Salvadoran Army officers accompany Duarte in the talks and a recommendation that neutral observers be present.

But a Salvadoran government official said he believes the conditions would not prevent the meeting from taking place. In addition, Duarte, without specifically mentioning the rebels' conditions, said this evening that he was going to the meeting as he proposed in his surprise offer yesterday in a speech to the U.N. General Assembly.

"I am going to the La Palma meeting with a feeling of agreement and peace," Duarte told about 1,000 supporters who greeted him on his return to El Salvador this evening at Ilopango military airport. "I am president of all Salvadorans," Duarte said.

The talks, if they take place, ideally would launch a process leading to a negotiated settlement of a conflict that has claimed the lives of more than 40,000 soldiers and civilians.

The United States has steadily increased its military support for the government despite criticism of El Salvador's human rights record, and U.S. policy here is an issue in the current U.S. political campaign.

Colombian President Belisario Betancur, now engaged in negotiations with his own country's insurgents, has been proposed by the Salvadoran left as an "intermediary" who could help them reach agreement with Duarte on arrangements for the encounter, announced Salvador Samayoa, a member of the political-diplomatic committee of the Farabundo Marti National Liberation Front in Mexico City. Rebel leader Ruben Zamora reportedly spoke privately with Betancur in Bogota today and said afterward that the Colombian chief of state "has committed himself to help" arrange the "concrete procedures" of the proposed negotiating session.

Duarte's offer to hold the talks represented a dramatic reversal of his previous policy -- which had been against immediate negotiations -- and thus marked a significant opening. The rebel movement repeatedly has urged negotiations "without conditions."

But the two sides have been far apart in the past when efforts were made at lower levels to hold negotiations, and rebel officials expressed considerable skepticism over whether Duarte was making his offer in good faith.

A Salvadoran official who supported Duarte's initiative said that the president's move was "impulsive" and that there was little preparation here before the offer was made. Duarte made the offer in part because "he had to say something special" at the United Nations, the official said.

In his offer, Duarte invited rebel military commanders to meet with him Monday at 10 a.m. in La Palma, a town in northern El Salvador that has alternated between government and guerrilla control. He proposed that both sides come without arms.

A communique issued today by the rebel alliance -- formally known as the Faribundo Marti National Liberation Front-Democratic Revolutionary Front -- announced "its acceptance of the meeting at the place, date and hour proposed." The communique was broadcast over the rebels' two clandestine radio stations and was distributed by civilian rebel officials in Mexico City and Nicaragua.

One potential snag was that the communique said two military commanders and two civilian leaders had been designated as the rebels' representatives in the talks. Duarte had said he was inviting only rebel military leaders to the meeting because the civilian leaders in the Democratic Revolutionary Front lack "authority to reach decisions."

The rebels demanded that Duarte be accompanied by members of the Salvadoran high command so that both sides would have civilian and military representatives. This condition was seen as a potential problem because Duarte frequently stresses that he is commander-in-chief of the armed forces and thus speaks for the Salvadoran military. The guerrillas have portrayed Duarte as a puppet of the military and of the United States.

The Associated Press reported that Secretary of State George P. Shultz is to meet with Duarte in San Salvador on Wednesday. Shultz will make the stop en route to Panama to attend the inauguration Thursday of President-elect Nicolas Ardito Barletta.

Broadcast this morning in El Salvador on Radio Venceremos, the rebel communique further proposed the evacuation of both leftist and Army troops from a 12-mile-wide area surrounding La Palma.

Yesterday Radio Venceremos had responded critically to Duarte's invitation, saying it was "not a serious proposal." Sources close to the rebel leadership here, however, said the radio station's remarks were simply "editorial comments" not reflecting the guerrillas' official position.

Today's rebel statement also requested that witnesses attend the meeting, "even though procedural issues will be handled privately." Earlier closed-door talks with a government peace commission collapsed after Salvadoran officials acused the guerrillas of "intransigence," Samayoa noted today. He added that without the participation of neutral observers "it was impossible for us to prove who it was that was being intransigent."