A halt in U.S. aid has sharply curtailed operations by the main anti-Sandinista rebel force at a moment when growing support among Nicaraguan peasants has swollen its ranks to about 14,000, according to the rebel military commander.

Former colonel Enrique Bermudez, who played a key role in founding the guerrilla movement and from the beginning has been its operational leader, said only a quarter of his irregular troops have been fighting. He said this was for lack of ammunition and boots and that it was a consequence of the U.S. aid cutoff taking hold during the past eight months.

His assessment, given in interviews Friday and yesterday at his headquarters on the Honduran-Nicaraguan border, contrasted with assertions by the rebels' civilian spokesmen that the guerrillas were continuing full-steam against the Popular Sandinista Army -- this despite the halt in funds and logistical help from the CIA ordered by Congress last spring.

"The object was to survive," Bermudez said at his sprawling military headquarters of the Nicaraguan Democratic Force, or FDN. "We constantly need ammunition, for AKs AK47 assault rifles , FALs Belgian-made assault rifles , grenade launchers, everything."

The FDN leadership authorized a rare visit to the camp by several American reporters on the condition that the camp's name and exact location remain secret. The visit coincided with efforts by the Reagan administration to generate support in Congress for resumption of CIA funding of what are known in Nicaragua as counterrevolutionaries, or contras.

President Reagan made an appeal yesterday in his weekly radio address for aid to "our brothers," the anticommunist guerrillas.

Bermudez said the money shortage was particularly frustrating because it struck as discontent among Nicaraguan peasants pushed growing numbers of youths to join the rebel movement. He said the force had grown from about 8,000 at the beginning of 1983 to about 14,000.

"The funding crisis was especially hard-hitting because we grew at the same time," he said.

Interviews with youths in training, most of them peasants hardened by life in the rugged Nicaraguan backlands, indicated that they left their villages to join the rebels principally because Sandinista authorities were trying to draft them into the Army.

The Nicaraguan government instituted a draft, called Patriotic Military Service, in October 1983 to help raise the Army to a strength estimated by Managua-based diplomats at 50,000.

Francisco Lagos, a 24-year-old peasant who joined the FDN seven months ago, just finished a two-week course in how to set and deactivate antipersonnel and antivehicle mines. A veteran of several skirmishes, he said he plans now to return to the hills and resume ambushing Sandinista trucks, a favorite rebel tactic.

"The Sandinistas don't let us work in peace," he said, explaining why he left his village of El Coyol near the town of Esteli. "You can't go to any town, because they will grab you and put you in the Army by force. I am a peasant; I don't want to be in any army. They are trying to get us all into the Army."

Asked why, then, he was willing to fight with the rebels, the copper-skinned man offered several ill-defined references to "communism" and Sandinista interference with traditional peasant independence. Then, like many of his comrades, he said fighting with the rebels was different because it was "voluntary."

Recruits of the other main non-Indian rebel movement, a small group headed by former revolutionary hero Eden Pastora along Nicaragua's southern border with Costa Rica, gave similar reasons in recent interviews for fleeing their villages.

Pastora's forces and the Miskito, Sumo and Rama Indians of Misura and Misurasata, rival Indian groups, have been fighting guerrilla wars similar to the FDN's. But Bermudez, who has built up fruitful U.S. military and intelligence contacts, received the bulk of the estimated $80 million spent by the CIA on Nicaraguan rebels between 1981 and the fund cutoff last spring.

The effect of the funding, along with Bermudez's conventional military background, was visible in this highly organized camp with its U.S. Army-style tropical huts, three-tiered security perimeters and 24-hour communications and radio monitoring services.

Adolfo Calero, the overall FDN political leader and chief of its fund-raising efforts, said in a recent interview that the organization has gathered a little more than $5 million from private American and other donors and "political sectors" abroad since the CIA money ran out. He denied published reports from Washington that other governments, such as Israel and Taiwan, have helped take up the slack.

Under urgent administration pleading, Congress voted last fall to allocate another $14 million to the rebel cause on the condition that the decision be reaffirmed in a separate vote this spring. With an eye to this second vote, administration officials and the FDN have sought to build up public and congressional favor for the guerrilla war.

Bermudez said Calero's efforts have brought in a recent dose of funds after a dry period, providing what the former National Guard colonel said was enough supplies for three more months. The main gaps now being filled, Bermudez and his lieutenants said, are boots for new recruits and rifle ammunition.

An indication of the boot shortage came at a ceremony for recruits who had finished training. Among the young men in khaki, with rifles on their shoulders, was one passing in review barefoot.

The rebels said new funds have underwritten purchase of some antiaircraft missiles for use against Sandinista planes and helicopters.

"We are working now on the following three months," Bermudez said. But he added, "The U.S. aid is necessary, vital."

Congressional refusal would not mean the end of the guerrilla war, he vowed, while acknowledging that Calero's fund-raising cannot go on indefinitely at a high level without at least the promise of renewed U.S. government funding.