SAN SALVADOR, AUG. 28 -- Leftist guerrillas proposed a five-day cease-fire today to facilitate peace talks next month with the government of El Salvador.

A statement broadcast by Radio Venceremos, the clandestine rebel station, expressed the guerrillas' strongest support yet for a regional peace plan signed Aug. 7 in Guatemala by the five Central American presidents.

"We recognize that the agreement by the Central American presidents, while it is a generic agreement made exclusively by the governments, establishes a general framework that is favorable for the search for political solutions to the internal conflicts in each Central American country," the statement said.

It suggested a Sept. 12-17 cease-fire "to create a better environment" for talks Sept. 15, the date President Jose Napoleon Duarte has suggested for the meeting. The statement was issued in the name of the Farabundo Marti National Liberation Front and the guerrillas' unarmed political wing, the Revolutionary Democratic Front.

Duarte proposed the talks in a speech Aug. 13 and the guerrillas accepted the next day. Some analysts have expressed doubt that the talks would actually take place because Duarte made them conditional on the guerrillas' endorsement of the peace plan and said the rebels must renounce violence as a means of taking power.

Duarte's U.S.-backed government did not respond immediately to the guerrillas' cease-fire proposal. A similar proposal was rejected last year by the Salvadoran armed forces, and a plan for government-guerrilla talks fell through. The two sides, locked in a civil war since early 1980, have had no formal talks since November 1984.

The guerrillas accepted the government's proposal Aug. 14 in a letter sent through the Roman Catholic archbishop of San Salvador, Arturo Rivera y Damas, but they have had no response. They suggested as a site for the talks the office of the papal nuncio in San Salvador.

"Since we have not heard an answer from Duarte," the guerrillas said in their statement, "we repeat our willingness to attend on Sept. 15 to discuss a cease-fire and to work on agreements that will lead to a national reconciliation and to resolution of the conflict through dialogue and negotiation."

The peace plan signed by the five presidents calls for cease-fires throughout the region by Nov. 7.