Nicaraguan exile leader Eden Pastora has called a unilateral cease-fire in his three-month-old guerrilla war against the Sandinista government following U.S. rejection of appeals from his movement for covert financial assistance.

According to Pastora's aides and co-directors of his Revolutionary Democratic Alliance (ARDE in its Spanish acronym), Pastora on Wednesday night told his troops via radio in southern Nicaragua to stop fighting, at least temporarily, while the leadership undertakes a "reevaluation of our resources . . . our means of assistance and a look at the real possibilities for moving ahead."

ARDE officials reached by phone in Costa Rica said that their troops will remain in place inside Nicaragua until the "reevaluation" is completed.

The decision was reached, they said, immediately following a conversation via radio telephone between Pastora, at his camp in Nicaragua, and ARDE political director Alfonso Robelo, in Washington. Robelo arrived here last week for meetings about U.S. assistance with Richard Stone, President Reagan's special envoy to Central America.

ARDE sources said Robelo had planned to ask for a minimum of $200,000 to $300,000 a month to keep their movement alive and supply what they say is a rapidly expanding force of 4,000 guerrilla troops, half of them "without uniforms, boots, food or arms."

"We never even got to numbers," Robelo said with apparent exasperation yesterday of his conversations with Stone. The Americans "say they have problems with the Hill, with the budget. They don't say yes or no, they just don't respond."

In response to questions yesterday concerning the ARDE request, a State Department spokesman said the department "does not comment formally or informally on allegations having to do with alleged covert activity."

Pastora's decision to ask for direct U.S. financial aid, the administration's failure to express interest in the proposal, and ARDE's subsequent decision to suspend military operations could mark significant changes in the Nicaraguan equation.

While posing little apparent military threat to Sandinistas so far, Pastora claims to control most of an area 65 miles long and 30 miles wide in southern Nicaragua along the Costa Rican border from the town of El Castillo to the Pacific. His troops, 2,000 of which he says are armed at least with rifles, have engaged in near constant skirmishes with Sandinista forces for the past three months. They admit, however, that the sparsely populated area is of little military importance.

Perhaps more significantly, Pastora is believed to be a popular figure in Nicaragua. He was a hero in the 1978-79 war that defeated the National Guard of dictator Anastasio Somoza. Known in Nicaragua as "Commander Zero," he left the Sandinista government in 1981 because of what he described as growing Cuban influence on the Sandinistas and increasing authoritarianism.

The Sandinistas have tried to discredit Pastora by saying he is tied to the United States and the separate guerrilla army of exiles it backs in northern Nicaragua. Ex-guardsmen lead that group. In response, Pastora publicly refused all U.S. assistance and ties with the northern exile group, known as the Nicaraguan Democratic Force (FDN).

The Reagan administration was criticized on Capitol Hill for its use of ex-soldiers of Somoza in a "secret war" against the Sandinistas. The government said aid to the FDN was designed only to help interdict a flow of arms through Nicaragua to guerrillas fighting the U.S.-backed government in nearby El Salvador. In the view of some officials, one way to quell the criticism was to join the two guerrilla movements under a joint leadership that would include Pastora, with his credentials as both an anticommunist and fighter against Somoza.

Pastora rejected such a plan and instead appealed to Latin American and Western European socialist and social democratic governments and parties for aid. ARDE attributes their failure to respond to an inability to stand up to the United States or the Sandinistas, despite their public unhappiness with the policies of both.

According to ARDE sources, the only substantial money they have received has come from private sources in Europe, Venezuela, Mexico and Colombia--money which some of them assumed had originated at least in part from the CIA.

But Pastora now says it has not been enough and he can no longer supply what he claims is a rapidly growing army with weapons or food.

In addition, Robelo said, the government of Costa Rica, whose northern districts had been relatively open to Pastora as a base for resupply and training, now has cracked down on ARDE's activities for fear of further involvement in the Nicaragua's problems. Three weeks ago, he said, the Costa Ricans confiscated ARDE's sole helicopter.

"As responsible people, aware of all these difficulties, we can't put our people into a situation of virtual suicide" against the Sandinistas, Brooklyn Rivera, one of the four ARDE directors, said by telephone from Costa Rica yesterday.

Yet, now that Pastora has decided to ask the Reagan administration for help, ARDE officials maintain, the administration is not interested. ARDE officials said they have considered several reasons for the rejection, including what they say is their insistence that aid have "no strings attached--no Green Berets or American advisers."

The administration "keeps assuring me that is not the case," Robelo said. He suggested another reason may be a "disadvantage" in fighting in southern, as opposed to northern, Nicaragua. The administration, he said, "cannot use the excuse that this is for arms interdiction."