SAN SALVADOR, JAN. 13 -- Five months after signing a historic peace plan aimed at ending the region's civil wars, a halt in El Salvador's bloody conflict is as far away as ever, according to government officials, diplomats and analysts.

"If the central purpose of the plan was to bring peace here, and it was, it did not happen," said a Western European diplomat. "All the rest of it is basically window dressing."

But officials said President Jose Napoleon Duarte has succeeded, to a degree at least, in isolating the Marxist-led Farabundo Marti National Liberation Front (FMLN), in the international arena.

The plan also allowed the FMLN's political allies, the Democratic Revolutionary Front to return to the country and begin open political work, strengthening Duarte's democratic credentials. But they also said they see the war as likely to grind on for many more years and that the chasm separating the two sides has widened.

In addition, opposition parties have abandoned the National Reconciliation Commission, saying they were not being taken into account -- further polarizing the political spectrum.

The plan, signed Aug. 7 in Guatemala, calls for a cutoff of all outside aid to insurgent groups, including U.S.-backed Nicaraguan contras and the FMLN; full democratic rights in all countries; amnesty for rebels who lay down their guns and for political prisoners; and cease-fires in countries with armed conflicts. The presidents are to meet Friday in Costa Rica to review the progress since the accord was signed.

Initially, hopes soared in El Salvador. According to polls taken in November, the popularity of Duarte, a staunch U.S. ally, shot up. But peace talks between the FMLN and the government soon broke down.

The FMLN broke off the talks to protest the killing of a prominent human rights worker. Last week, Duarte presented a witness who said he belonged to the FMLN and that the guerrillas carried out the attack.

A poll of 1,080 adults taken in September showed 86 percent of the people had heard of the peace plan, an unusually high figure in a poor nation with only a 50 percent literacy rate.

Duarte has said El Salvador has completed every phase of the plan and that at the summit it will point to the government's 15-day unilateral cease-fire, a sweeping amnesty that freed 470 accused or convicted leftist rebels, organization of an internal commission on dialogue and maintenance of press freedoms.

According to three sources who know Duarte well, he has also shut down all contra resupply efforts out of El Salvador.

Critics charged that the cease-fire was illusory, with the Army fanning out across the country in what amounted to a large-scale offensive.

Human rights groups charged that the amnesty went far beyond what was called for in the pact, also closing the books on tens of thousands of unsolved death squad murders and making it impossible to prosecute the military for killings of civilians.

According to senior officials, the United States, interested in embarrassing Nicaragua, originally supported the amnesty. But when judges ruled that the convicted killers of two American land reform advisers were pardoned, the United States reacted angrily, promising to review aid to the judiciary.

"They wanted a broad amnesty they could hit Nicaragua over the head with, and when they got it and realized the consequences, they started screaming," said an adviser of Duarte.

The Reagan administration has sent high-level delegations to its four allies in the region, including the national security adviser, Lt. Gen. Colin Powell.

"Their message was that they want the presidents of El Salvador, Guatemala, Costa Rica and Honduras to condemn Nicaragua for not fulfilling the plan, and support contra aid," said a person who attended the meeting with Powell. "They argue that without contra aid, these countries will face the aggression of Nicaragua by themselves."

While Duarte has said he will take the lead in condemning Nicaragua's failure to comply, he will not back contra aid, according to advisers. "Duarte cannot support contra aid without legitimizing aid to the FMLN, so what can he do?" one said.

The implementation of the plan also exacerbated tensions between Duarte's civilian government and the powerful military. Perhaps the sorest point was the decision to allow leftist political leaders to return. Top officials said they thought the return was a "Trojan horse," allowing the FMLN to develop both a military and a political front.