SAN SALVADOR -- Salvadoran rebel leaders, due in Costa Rica for further peace talks today, are facing criticism within their own ranks and from leftist civilian organizations who say they were sold short by a human-rights accord reached with the government last month.

The accord, the first of its kind in 10 years of civil war, has opened up a heated internal debate over rebel strategy at future rounds of negotiations. Rebel leaders have since called for a key provision of the accord to be changed. They want a United Nations human-rights verification mission to be set up immediately rather than after a cease-fire.

A visit to the mountainous northeastern town of Perquin, long a stronghold of the Farabundo Marti National Liberation Front (FMLN) rebels, showed that some of the guerrilla fighters question whether the accord signed in Costa Rica provides the guarantees for the changes they want.

"In terms of what is needed to end the war, it's not much," said Ernesto, a veteran mid-level commander. "We shouldn't sign superficial agreements. We should sign things that come to a real political solution."

The dissent highlights the constraints placed on rebel leaders by a radical rank and file, many of whom joined the guerrillas after family members were killed and who fear a similar fate after a peace settlement.

In past years, the shells of destroyed houses in Perquin were daubed with slogans calling for revolutionary victory over the rich. But these have been replaced, in tune with the change in rebel strategy toward a negotiated settlement. "The FMLN is the guarantee of democratic change in El Salvador," reads a typical new slogan.

Rebel fighters appeared confident of their military strength after their massive offensive of last November. Some argue that further concessions to the government are unnecessary. The rebels have been threatening a new offensive, should talks fail. There is an air of confident expectancy among their forces.

Senior commanders of the Salvadoran army contend, however, that the rebels did their utmost in November. With both sides suggesting they can gain more on the battlefield, diplomats and other observers are not optimistic about the short-term prospects for peace.

The human-rights accord was signed, under strong pressure on both sides from United Nations mediator Alvaro de Soto, after talks deadlocked on the central rebel demand for a radical restructuring of the U.S.-backed army, including a far-reaching purge of the officer corps. The government says a purge is out of the question.

The human-rights agreement binds both sides to immediate measures to prevent violations, in particular of the right of individuals not to be killed, seized without reason or explanation, tortured, mistreated or held incommunicado.

But verification by a U.N. commission will come into effect only after a cease-fire. That provision has provoked criticism from rebel ranks -- and from leftist unions, student organizations and other groups -- that the agreement has no teeth.

"What is going to happen meanwhile?" asked Oscar Hernandez, a leader of the nongovernmental Human Rights Commission. "The violations will continue. We want a peace with justice, not a peace of institutionalized violence."

U.N. officials are thought to oppose immediately setting up a verification commission for practical reasons. "It's not something that can be done overnight," said one diplomat close to the United Nations. "If it's going to be done it has to be done properly. Otherwise it will not provide any guarantees."

The rebel leaders' strategy was heatedly defended by a senior commander with the nom de guerre of Fito, who described the agreement as a "victory" and said it showed a tacit admission by the army that it has been violating human rights.