The sudden new burst of guerrilla activity against Nicaragua's government this month is the result of a new strategy imposed on the Nicaraguan rebels by the Central Intelligence Agency that funds them, according to U.S. and rebel sources interviewed in recent weeks.

For the Nicaraguan rebels the revitalized offensive this month is said to represent a "do or die" effort aimed as much at convincing the CIA and the U.S. Congress that they are worth the money put into their operations as at pressing the Sandinistas in Managua.

According to sources here, in Honduras and in the United States, the initial ineffectiveness of the CIA-orchestrated anti-Sandinista campaign begun last year produced an ultimatum. With Congress threatening to cancel funding for the not-so-secret "covert" operations, the CIA is reported to have told its leading Nicaraguan surrogate group, the Honduras-based Nicaraguan Democratic Force (FDN in Spanish initials), last spring that unless it "shaped up," funds would be cut off by this September.

The date was important because it represented the time when Congress would once again have to act on renewing covert funding for the coming fiscal year.

According to a senior FDN official interviewed by Washington Post correspondent Edward Cody this week in Miami, so serious was the CIA pressure that, by April, supplies to their forces in the field had dried up--leaving units deep inside Nicaragua without the support imperative for their survival.

"We were left with no choice but to call our people back to their bases in Honduras," the FDN official said. "Our people in Nicaragua were in such bad shape that some of them straggled back to their bases literally without shoes."

The official said this explained the summer lull in anti-Sandinista activity that ensued, because it takes up to 30 days to move in or out of operational bases in Nicaragua.

A well-informed U.S. source in Tegucigalpa, the Honduran capital, said the recall of the FDN forces was not only the result of their sudden lack of U.S. supplies, but because of the insistence of the CIA station chief there that they come up with a credible strategy that could convince "doubting Thomases" in Washington of their viability against the Sandinistas.

The disastrous attempts of the guerrillas to capture Nicaraguan border towns, their petty harassments of Sandinista militia units and civilians in the countryside, and the estimated $70 million in economic damage inflicted by sabotage by this summer, were deemed by U.S. officials to have failed to present the Sandinistas with a serious military or political threat.

"Our effort to simply fight a war in the rural countryside was a failure because it did not produce the popular uprising of the Nicaraguan people we had originally hoped for," said a senior FDN leader in an interview recently. "It was brought home to us that we needed a more dramatic show of force to convince the world of our seriousness. It was decided that to do this we would have to carry our struggle into the cities of Nicaragua as well as its isolated countryside."

"We became convinced that if we don't get into the cities," said Edgar Chamorro, a senior FDN leader, in a recent interview in Tegucigalpa, "then we are doomed to fight for another 20 years in the mountains."

According to FDN sources, the general reevaluation of strategy was completed by the end of June. In early July, with renewed U.S. supplies, the FDN began the arduous effort of reinfiltrating its units--variously estimated to comprise from 4,000 to 7,000 men. According to the same sources, at least one C47 transport and smaller civilian aircraft were used to parachute additional supplies to the forward units after they began arriving at their bases late this summer.

"Our strategy now has been to go for important economic targets that would hurt the Sandinistas," a senior FDN leader said, "as well as to make our struggle known in urban communities by briefly occupying them, painting their streets with slogans, holding a political rally or two, distributing arms to sympathizers for later use, then withdrawing before the Sandinistas could mobilize to concentrate their superior forces against us."

The CIA, according to these sources, also urged new efforts to unite the various anti-Sandinista factions--especially the FDN and the Costa Rica-based Democratic Revolutionary Alliance (ARDE), commanded by dissident Sandinista hero Eden Pastora.

Pastora, who made his name with the Sandinistas fighting to overthrow dictator Anastasio Somoza in 1979, has refused to unite his forces with the FDN because it was commanded by Somoza-era national guard officers.

Despite repeated hints by FDN officials that unity or cooperation with Pastora's group is at hand, his spokesmen here remain adamant that so long as national guardsmen remain in key leadership roles in the FDN, there can be no unity.

Despite Pastora's disclaimers, however, there are signs that he is no longer operating in the same isolation as when he began his struggle against the Sandinistas May 1 in the sparsely populated southeast corner of Nicaragua that adjoins Costa Rica.

ARDE initiated air raids against Nicaragua with a makeshift bombing of Managua's international airport Sept. 8, coinciding with a FDN commando raid against the vital Nicaraguan oil terminal at Puerto Sandino. Although his supporters have denied there was any coordination, the FDN briefed journalists in Tegucigalpa within hours of the two raids, claiming that they had previous knowledge of Pastora's bombing raid.

ARDE officials admit they have had "mysterious" infusions of money this summer from foreign sources--of whom they chose not to ask too many questions. This has raised the suspicion that, all official denials aside, ARDE has joined the FDN in a CIA-inspired guerrilla offensive against Nicaragua.