SAN SALVADOR, AUG. 20 -- El Salvador has presented a proposal to a meeting of Central American foreign ministers here indicating it is willing to see an end to U.S. aid to the Nicaraguan rebels if it can be assured that Nicaragua and Cuba will end all support for the Salvadoran guerrillas.

In the 11-page proposal, El Salvador elaborates on its approach to a peace plan signed Aug. 7 in Guatemala by the five Central American presidents, based on a proposal by Costa Rican President Oscar Arias.

The Salvadoran document reaffirms the Guatemala accord but goes beyond it, revealing a shift in El Salvador's position since its military in 1986 hosted an air resupply operation organized by former National Security aide Lt. Col. Oliver North to help the Nicaraguan rebels, or contras.

The proposal indicates that Salvadoran President Jose Napoleon Duarte sees the Guatemala plan as a way to bring new international pressure on the Salvadoran guerrillas to give up their seven-year-old war, while cutting off all their outside aid. It suggests that if Duarte can achieve some respite from the fighting at home, he would not object if U.S. aid to the contras became a casualty.

Duarte is scheduled to meet Friday with the six leaders of the Nicaraguan Resistance, the main contra alliance.

Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica still must overcome obstacles to put the pact into effect Nov. 7, 90 days after its signing, as agreed. El Salvador's proposal has yet to be approved by the five nations.

The five foreign ministers today ended a two-day parley, the first step toward carrying out the Guatemala plan. The ministers, including Nicaraguan Miguel d'Escoto, all said they were satisfied with the results.

But according to a seven-point communique, their main advance was to ask leaders of the United Nations, the Organization of American States and the Contadora group of Latin American nations to form a commission to make sure the agreement is respected.

The Guatemala plan calls for full democratic freedoms in all countries, an end to outside aid to guerrilla movements, political dialogue between governments and their civilian opponents, and a regional cease-fire.

The proposal by Duarte's Christian Democratic government stresses the need for "irregular or insurgent groups" -- such as the leftist Salvadoran guerrillas and the rightist contras -- to lay down their arms and "enter into the democratic process."

The document also proposes that the governments jointly issue a call "to irregular groups operating in other countries to lay down their arms and enter into a cease-fire."

It rejects all "material, logistical, moral, financial or political aid" to rebels in the region and says the Central American governments should refrain from providing it and jointly call on governments outside the region to stop all such aid.

The proposal urges the Central American governments to give the international verification commission evidence of such aid and to punish anyone who provides it by impounding planes, trucks and boats used to carry it. The document mandates a more aggressive role than the Guatemala plan for international inspection.