In a highly unusual interview, the key military commander of El Salvador's leftist rebel forces, Joaquin Villalobos, has told a delegation of six Americans that he fully supports peace negotiations with the government.

Villalobos met for three hours yesterday with a former U.S. congressman from Massachusetts, James Shannon; an aide to Rep. George Miller (D-Calif.) and four other Americans organized by the Commission on U.S.-Central American Relations, a liberal, Washington-based policy watchdog group. They were accompanied by a reporter for Agence France-Presse who did not attend their meetings with Villalobos.

Villalobos, apparently reclusive and security-conscious, has never permitted an American journalist to interview him inside El Salvador during nearly five years of war between the government and the guerrillas' Farabundo Marti National Liberation Front.

His failure to appear Oct. 15 at the first round of peace talks in La Palma, and the assassination shortly afterward by his men of the government's most skillful combat commander, Lt. Col. Domingo Monterrosa, gave rise to speculation that Villalobos wanted to win the war rather than settle it at the negotiating table.

Villalobos asserted that he and the four other senior commanders of the Marti front, as well as its diplomatic representatives in exile, all agree on the need for peace talks, the U.S. group reported. The talks have been suspended since a second round at Ayagualo Nov. 30.

The guerrilla leader said he believes a cease-fire could be negotiated with the government if the United States ended all its aid to the Salvadoran armed forces.

The delegation members did not carry a tape recorder to the interview and could not reconstruct from their notes the exact phrases Villalobos used.

The meeting, organized at the guerrillas' initiative, took place in Perquin, a rebel capital in northeastern Morazan province.

On April 5, a government spokesman, Maj. Carlos Aviles, circulated a report he said was based on Army intelligence that Villalobos had been killed in combat March 31. Four days later Villalobos went on a rebel radio broadcast to signal that he was still alive. Yesterday he appeared "fit and healthy," Shannon said.

Asked about his political beliefs, Villalobos said he is "sympathetic to Marxism, but not a Marxist in a disciplined way," said Peter Bell, an associate of the Carnegie Endowment for International Peace in Washington. Later Villalobos told the French reporter, "We are not Marxists; we are Salvadoran revolutionaries."

While "extremely optimistic" about the guerrillas' progress in the war, according to Miller aide Cynthia Arnson, Villalobos did not say they could take a military victory. "We are a veto force" in any negotiation, Arnson quoted him as saying.

It was unclear why Villalobos came out of his self-imposed hiding to see the group. "It was clear he wasn't trying to send any messages through us," Bell said.

Villalobos acknowledged that the recent legislative elections, in which Christian Democratic President Jose Napoleon Duarte won a sweeping majority, could improve conditions for peace talks if they mean the Army and the landholding wealthy will enjoy less power.

Villalobos is the top commander of the People's Revolutionary Army, one of five rebel armies in the Farabundo Marti front, which operates in eastern El Salvador.