SAN SALVADOR, AUG. 19 -- A meeting of Central American foreign ministers opened today to begin the thorny task of implementing a preliminary peace accord signed in Guatemala by the region's five presidents earlier this month.

As a sign of the difficulties the peace process may face, the Honduran foreign minister, Carlos Lopez Contreras, did not attend. A Honduran representative said an "inconvenience" had delayed his travel plans. But diplomats from the other countries perceived Honduran hesitation about the plan.

The agreement signed Aug. 7 by the presidents of Guatemala, El Salvador, Honduras, Nicaragua and Costa Rica called for a meeting of foreign ministers within 15 days to start ironing out difficult details. The results of the two-day session will give the first indication whether the five countries, divided on several important issues, can continue to talk productively and reach agreements, or will get bogged down in diplomatic disputes.

The ministers' main agenda item this week is to appoint commissions with members from the different countries to focus on specific problems in fulfilling and verifying the terms of the peace pact.

The plan, based on a proposal by Costa Rican President Oscar Arias, calls for full restoration of democratic freedoms, a clause which points mainly at leftist Nicaragua. It also calls for an end to foreign aid to rebel forces in the region and bars guerrilla operations from the countries against their neighbors.

It calls for dialogue between the governments and their civilian political opponents, and would culminate in a regionwide cease-fire to begin Nov. 7.

In the two weeks since Aug. 7, many obstacles to implementation of the plan have come to light. Today the foreign ministers began to note the consequences for the region if the accord collapses.

"The document is only the beginning of an arduous set of steps we must take," said Salvadoran Foreign Minister Ricardo Acevedo Peralta. "If we fail, we face international rejection and disdain and the end of our credibility in the eyes of history."

In a sign of Honduras' reluctance about the plan, Lopez Contreras said in an address to the National Assembly in Tegucigalpa yesterday that the accord is a "recommendation" and not binding like a treaty, according to radio reports in Honduras. Some legislators there have complained they had no chance to ratify the plan.

A military officier in the Honduran delegation, responding to the clauses barring the five nations from allowing guerrillas to operate from their territory, reiterated his country's denial that the U.S.-backed Nicaraguan rebels maintain any base camps on its soil. Although most of an estimated 10,000 rebels, known as contras, are in Nicaragua, their commanders maintain strategic command and air resupply operations in Honduras.

The Honduran government anticipates that thousands of contras may move northward out of Nicaragua and back to their bases in Honduras this fall, according to the officer. Honduras expects Nicaraguan Army to step up the pace of fighting to maximize its advantage before the cease-fire, he said, and the truce could unleash an additional flow of contras out of Nicaragua. Honduran soldiers will not fire on contras fleeing peacefully from Nicaragua, he said.

Honduras also is worried that the contras will run out of food if U.S. aid stops flowing between Sept. 30, when the fiscal year ends, and Nov. 7, when the cease-fire would take effect, a diplomat said in Tegucigalpa.

In Managua, Vice President Sergio Ramirez issued an appeal to Latin American and other western nations for help in obtaining more oil for Nicaragua, which he said will run out of gasoline before the end of this year, according to press reports monitored here. Nicaraguan officials have said recently that the Soviet Union informed Nicaragua it would provide only 4 million barrels of oil this year, about 40 percent of its annual needs, which represents no increase over the Soviet contribution in 1986.

Ramirez said Nicaragua needs 1.6 million barrels of crude oil to finish the year, but has no funds to pay for it. Without some donation, he warned, Nicaragua will face "grave limitations in the functioning of its economy, and this could have repercussions on the active participation of Nicaragua in the peace process."

In an apparent attempt to use the peace process to seek help in alleviating Nicaragua's fuel shortage, Ramirez said, "a country that doesn't have its oil resources ensured for the rest of the year cannot have the security and tranquility to engage in a process of this magnitude."

One factor that has given impetus to the Guatemala pact, observers said, is the behind-the-scenes participation of U.S. Democratic legislators who see the accord as an alternative to the Reagan administration's backing for the contras.

"The Democrats are sending messages to the Central Americans that they are the future and the contras are the past," said William Goodfellow, director of the liberal Washington-based Center for International Policy, who is observing the meetings.

The six top leaders of the Nicaraguan Resistance, the main contra alliance, arrived here today for a meeting Friday with President Jose Napoleon Duarte.

{Contra leader Alfredo Cesar said in Washington that he and his colleagues will meet with President Reagan in California next week to present their concerns about the peace plan.}