MANAGUA, NICARAGUA -- One year after $100 million in U.S. aid for the Nicaraguan rebels was approved, they have carried the fighting to many corners of Nicaragua but remain far from challenging the Sandinistas' control, according to U.S. diplomats and military officers, rebel leaders and Sandinista commanders.

What the rebels, or contras, did in the past year, these sources said, was to complete the first phase of an unfolding prolonged insurgency. At the current pace, the contras will require U.S. aid at the same levels for years to come to be able to threaten Sandinista rule, sources on all sides said.

The contras' war is "definitely slow," said a top U.S. military analyst in Panama. "It's not static but it's not very dynamic. This type of war could persist for literally years."

"It's a long-term guerrilla war. Those are the tactics we are using," said a U.S. diplomat in the region who monitors the contras.

Ambitious predictions that contra leaders offered a year ago of using U.S. aid to provoke an uprising against the Sandinistas have been abandoned.

"It is very premature to talk about an insurrection. We haven't reached that stage," said the contras' spokesman Bosco Matamoros by telephone from Miami.

The contras have yet to chalk up a major, attention-getting battle success anywhere in Nicaragua, U.S. and western military observers noted.

The talk in Central America since Aug. 7 has been mostly of peace, when the region's five presidents signed an accord scheduling a cease-fire in November. But the rumors of peace have caused no slackening in the rhythm of war in Nicaragua.

Missile-firing contras downed a Soviet-made Sandinista helicopter Aug. 28, killing at least six soldiers, the Defense Ministry announced. The contras claim to have destroyed or damaged 14 choppers this year.

A Nicaraguan Army major and a captain were killed in an ambush in Jinotega province the same day, a defense spokesman said, adding that the major was the highest-ranking officer killed in the field this year. At the same time, the spokesman said, Sandinista troops seized a load of grenades and medicine that was air-dropped to the contras.

The $100 million, approved by the Senate in August 1986, started to flow in October. In December, the contras launched an offensive to move an estimated 10,000 reequipped fighters out of Honduras border base camps into Nicaragua. Administration officials are now deciding when to go to Congress for new aid, as this year's package expires Sept. 30.

In the past eight months the contras, operating in light patrols of two dozen or so fighters, have moved down Nicaragua's backbone of mountains. Ambushing Army convoys, skirmishing with Sandinista jungle platoons, assaulting agricultural cooperatives, the contras have made their presence felt from northern Jinotega province to Zelaya in the south.

In June, for example, there were at least five combat incidents across the country daily, according to contra accounts confirmed by independent reports.

"We are reactivating," said the contras' top military commander, Enrique Bermudez, in an interview. "We are regaining terrain we lost." During 1985-6, when U.S. aid was cut off, contra forces spent most of their time training in Honduras camps.

"They infiltrated 10,000 guerrillas into Nicaragua with near impunity," said the U.S. diplomat.

"We did not fall into the trap of frontal engagement. We outflanked them," explained Matamoros.

Military observers say the contras maintain the initiative in the limited sense that they decide where and when to attack, forcing the 65,000-troop Sandinista Army to spread itself thin. They are demonstrating new skills, apparently learned from Green Beret and CIA trainers, at clustering forces for actions and then scattering before Sandinista reinforcements arrive.

"A war of attrition has taken on new importance," acknowledged the Sandinista deputy defense minister, Major Gen. Joaquin Cuadra, in a recent interview. Instead of a large force, he said, the contras "are trying to develop a highly trained force which avoids head-on clashes with Sandinista troops and seeks to move fast and disperse widely across our territory."

However, the offensive has not been able to get past Sandinista defenses to move down from the mountains to populous towns, let alone cities, or to strike critical objectives.

"They're still going after small, soft targets" like farmers' cooperatives, the U.S. military analyst said.

"With the amount of aid we have received so far, we haven't contemplated making the Pacific region a theater of operations," Matamoros said. Most of Nicaragua's cities, including the capital, lie on the Pacific coastal plain.

In June, a contra unit made one foray toward the coast in Leon province, but was quickly driven back. Except for occasional electrical blackouts, there is no day-to-day feeling of war in urban areas.

Contra spokesmen eagerly billed a July 16 attack on the northern town of San Jose de Bocay as their most important triumph this year. But reporters found the contras failed to occupy the town, as they claimed, or to destroy its airfield. They killed six civilians, including a woman and three children. At least 13 Sandinista troops also were killed, but so were at least 15 contras.

Contra leaders advertised that sabotage against electric towers and fuel supply facilities would be a conspicuous feature of this summer's fighting. But after a string of attacks in June, the sabotage campaign faded when Sandinistas redoubled security at key points and captured at least one contra commando unit.

During the past year, the number of volunteers dwindled to almost nothing, contra commanders admit, while Sandinista forces, who are drafted, continued to grow. Cuadra said the Sandinistas can put up to 200,000 Nicaraguans under arms in a full mobilization.

A number of Sandinista soldiers in south-central Nicaragua, where the government is unpopular, deserted in recent months to join contra ranks, contra spokesmen and witnesses said. Contra commmanders admit, however, that a Nicaraguan amnesty, allowing rebels to return to civilian life, exacts a continuous drain on their forces.

Top contra field commanders pledged to continue their fight, with or without U.S. aid, but they conceded they have no hope of prevailing without it. They said the Sandinistas received Soviet bloc military aid worth $535 million last year.

Contras deep in Nicaraguan forests depend for supplies on CIA-run air missions, one aspect of contra operations that the Sandinistas do not appear to have interrupted.

A U.S. military officer in the region warned that if new aid is voted and there is no cease-fire, there will be a high risk of the Sandinistas finally importing advanced interceptor aircraft to shoot down the contras' flights.

In Nicaragua, Sandinista military leaders suppress much news about contra combat. But the pressure has not made them revise their confident rhetoric. Cuadra asserted that the contras' strength reached a peak that was dangerous for the Sandinistas in 1984, before the first U.S. aid cutoff, but that in the following two years the Sandinistas dealt the contras an "irreversible strategic defeat" and are now concerned primarily with upgrading their conventional capabilities to prepare for a direct attack by the United States.

"We don't expect to defeat the Sandinistas in battle. But we can create the conditions for their collapse," contra chief Bermudez countered. Contra commanders say the high cost of the war -- nearly half the national budget this year -- forced President Daniel Ortega to sign the Aug. 7 peace plan.

But if the peace efforts fail, U.S. military officers worry that the Sandinistas have the advantage in a drawn-out war, because the government can replace its casualties and supply its troops more easily over time.

Said the U.S. military analyst: "It looks like we have to be prepared to stick it out for three, four, five years. But the Sandinistas own the real estate. They can sustain the long haul much better than the contras."

Special correspondent Wilson Ring contributed to this story from Tegucigalpa, Honduras.