PANAMA CITY, AUG. 16 -- The leader of the Salvadoran rebels' civilian wing, Guillermo Ungo, said the movement will not accept all terms of a peace pact signed Aug. 7 by the five Central American presidents, and he doubted that the rebels would meet soon for talks with the Salvadoran government.

In an interview at his home here yesterday, Ungo responded at length to a call for a new round of peace talks issued Thursday in El Salvador by President Jose Napoleon Duarte. Duarte proposed meeting Sept. 15 to discuss a cease-fire mandated in the presidential accord. But he demanded that the rebels publicly accept the full pact before Aug. 30.

The signing of the accord in Guatemala raised hopes in this region exhausted by prolonged wars in El Salvador, Nicaragua and Guatemala. But in El Salvador, distrust and political posturing on both sides that have crippled past peace efforts quickly reemerged.

The Guatemala accord evolved from a plan presented early this year by Costa Rican President Oscar Arias that was intended primarily to press the leftist government of Nicaragua to allow more democratic freedoms. While the final document applies equally to all five countries, its terms fit El Salvador's seven-year civil war awkwardly. The terms were disputed on several fronts.

The tentative accord calls for a cease-fire within 90 days and for talks between governments and civilian opposition groups, but not armed guerrillas. It says the cease-fire must come within a "constitutional framework" but does not specify how it is to be negotiated.

Ungo said his movement sees the peace pact as a "positive contribution" but rejects some of its clauses: "We weren't in the Guatemala meeting. Why should we pay for the costs Duarte had to run up to be able to sign? Why should we put on a straitjacket that doesn't fit us?"

Ungo is president of the Democratic Revolutionary Front, a coalition of leftist parties allied with the Farabundo Marti National Liberation Front, which includes five guerrilla groups.

Duarte, in his nationally televised speech Aug. 13, called for the rebels to "join the democratic process," a phrase he has used in the past to demand that they lay down their arms. He called for the talks on Sept. 15 and asked the Nicaraguan government to meet with the Nicaraguan rebels, known as contras, on that day as well.

On Friday, the Salvadoran rebels responded by agreeing to go to a two-day meeting starting Sept. 15 in a Roman Catholic office in San Salvador, to dicuss not only a cease-fire but also a political solution of the war. Friday evening, Duarte rejected the rebels' response as "not sufficent" because the guerrillas did not in principle renounce armed warfare.

"We don't want to say this chapter is closed. But I'm not optimistic," Ungo said. The Salvadoran rebels, with an estimated 6,000 fighters inside the country, reject aspects of the Guatemala pact that put them on the same footing as the contras, he added.

"You can't put the same sailor suit on a boy of 6 years and on an adult of 30," Ungo said. "We have completely different and much greater decision-making power and self-sufficiency than the contras."

The contras received $100 million in U.S. aid over the past year, and the Reagan administration is expected to seek at least $150 million more sometime this fall. The Guatemala pact calls for an end to foreign aid for insurgent movements. The Nicaraguan government views the contras as mercenaries and has never held talks with them. It refused again last week.

In El Salvador, by contrast, a tortuous and so far fruitless process of peace talks between the government and the guerrillas began in the town of La Palma in September 1984. The talks collapsed after a rancorous meeting in Ayagualo two months later. An attempt to revive them in September 1986 also failed amid disputes about security.

The rebels argue that the presidential accord obliges Duarte to renew the dialogue. Duarte argues the accord obliges the rebels to give up the armed struggle and talk with the government about returning to civilian life.

Ungo said the rebels could comply with a negotiated cease-fire but saw no prospect that they would lay down their arms for good. Gen. Eugenio Vides Casanova, the Salvadoran defense minister, has said the Army would respect even a unilateral cease-fire if Duarte ordered it.

The pact's clauses stipulating an end to foreign arms aid to guerrilla groups "do us more good than harm," Ungo asserted. He claimed the weapons the Salvadoran guerrillas bring in from outside amount to a small fraction of what the contras get from Washington, and that foreign governments, primarily Cuba, are "hardly ever involved anymore."

Ungo alleged that the guerrillas had weakened the Salvadoran military and Duarte's government in the past two years, but not enough to tip the balance one way or the other nor to increase the pressure for negotiations.

Ungo, president of a small social democratic party, has been living in exile for seven years in a humid Panama City apartment, where reporters have gathered countless times under the whirling ceiling fans to probe the tangles of stalling peace talks.

Ungo's Democratic Revolutionary Front in recent years has played a risky political game by sending some members back to live openly in El Salvador while maintaining a link with the guerrillas.

Ungo said his group would continue to expand its work in El Salvador but is not considering any break with the guerrillas.

United Press International added from San Salvador:

Salvadoran Archbishop Arturo Rivera y Damas said that he, as intermediary, had received the rebels' response to a government proposal for peace talks, but warned that differences between the two sides could torpedo the talks before they start.

"I am afraid because it is an answer to the proposal the president made within the framework of the Guatemala document, and the rebels' response ignores that, and that is one of the main reasons the talks may fail," Rivera y Damas said.