A new package of $100 million in U.S. military aid to the Nicaraguan rebels is "a turning point" that will produce "a better-balanced war" against the leftist Sandinista government and "cracks in the Sandinista structure" within a year, a top rebel leader said yesterday.

Alfonso Robelo, one of three directors of the United Nicaraguan Opposition, told a news conference he also hoped for "popular insurrections" against the Sandinistas within Nicaragua and "massive defections" from the Sandinista Army that would produce "a more realistic possibility for a political solution."

The question of whether more U.S. aid will be needed to achieve this goal "depends on how much more aid the Soviet Union is prepared to give the Sandinistas," Robelo said.

The Reagan administration has not formally stated how the aid will be distributed but officials have made no secret of their intention to have the Central Intelligence Agency actively involved in helping the rebels, known as contras. Also, Gen. John Galvin, commander of U.S. forces in Central America, has said that he hopes to provide American soldiers who can train the contras in Honduras.

Of the $100 million package, $70 million is military aid, which will be the first time the United States has provided arms to the contras since March 1984. Before that cutoff, the CIA sponsored a number of controversial operations against the leftist Sandinistas, including the mining of Nicaraguan harbors and the destruction of a fuel storage depot at Corinto.

Administration officials hope the new infusion of aid will help the contras, who have been largely ineffective as a fighting force, become more aggressive and carry the war to Sandinista cities, including the capital of Managua. They also hope to revitalize the contra's currently moribund southern front operating out of Costa Rica.

In New York, Nicaraguan Foreign Minister Miguel D'Escoto said on NBC's "Today" show that the new aid was "a vote against peace, in favor of death." He said it will "cost many more deaths" of Nicaraguans, poses "a risk of escalation" because U.S. military advisers will accompany the contras, and "makes greater . . . the prospects of direct U.S. intervention."

He declined to say whether Nicaragua would seek more Soviet help, but said the Sandinistas "won't reject any aid . . . from any country."

Defense Department spokesman Robert Sims told the Pentagon press corps that the Soviet Union has recently doubled Nicaragua's arsenal of heavily armed helicopters and has sent test pilots and technicians there for the first time. He said recent Soviet cargo shipments included containers of helicopters, and that Nicaragua now has 25 Mi8 assault helicopters and 12 Mi24 Hind-D helicopters.

The Soviet Union has provided $650 million in military aid to Nicaragua this year, for a total of $2 billion since the Sandinistas took power in July 1979, according to Defense Department figures.

Knight-Ridder Newspapers last night quoted intelligence sources as saying that training is the first priority of the new program and that in some cases contra field commanders may be flown to U.S. military bases in Panama or the United States to attend officer training course. The Pentagon plans to send 40 to 50 Green Berets to Honduras as contra instructors, the report said.

The report also quoted a March 12 report from the House Intelligence Committee based on a secret March 4 CIA briefing which said, "Approval by the House of the president's request means a commitment to a force two or two and a half times its present size." The current contra force is usually estimated at 15,000 to 20,000.

The report also said the committee was told that if Congress approved Reagan's proposal, the CIA would expect the contras "to hold territory" and administer it.

In the March 12 report, the House Intelligence Committee said the goal of the new plan is not to overthrow the Sandinistas but to pressure them into negotiating with the contras, Knight-Ridder reported.

Although the Senate has not yet acted, House approval of the President's military aid request Wednesday had the immediate effect of smoothing out differences among the many factions of the rebels, even as it sparked defiance from the Sandinistas.

Robelo said that UNO plans to start building a government-in-exile by convening representatives of 16 anti-Sandinista groups early next month to reach a consensus on UNO leadership. The new aid "has given us a tremendous responsibility, and we want to see all Nicaraguans on board," he said.

Jose Cardenal, a former rebel leader who was ousted in 1982 and thereafter condemned as a "Somocista," or supporter of deposed dictator Anastasio Somoza, appeared at the news conference to rejoin the movement. Earlier this week he said all three UNO leaders were corrupt and should be replaced.

Similarly, leaders of the small, independent Southern Opposition Bloc of rebel groups -- who also had exchanged harsh words with UNO leaders -- announced they had signed an agreement of "coordination" with UNO on June 18 in Costa Rica. "We have the assurance that aid will be distributed fairly for everybody," said Alvaro Jerez, a leader of the bloc. "The time for factionalism and political interests should now be set aside."

Adolfo Calero, another UNO leader, said the 20,000 rebels -- also known as contras or counterrevolutionaries -- are prepared for new Sandinista attacks in the next few weeks. He said the "small weapons" that may be provided after Sept. 1 and before next February under terms of the House bill should include grenade launchers, machine guns and ammunition as well as shoulder-fired antiaircraft missiles.

The administration has said it is considering providing the advanced Stinger shoulder-fired missile to the contras and to other insurgent groups. At the Pentagon yesterday, Sims said "as to the specifics of what might be in" the military package sent to the contras, "I have no idea."

The new U.S. aid includes $27 million earmarked for food, medicine, clothing and other nonlethal aid, $70 million for military equipment and $3 million for a new UNO human rights office. UNO also plans to set up a six-member "blue-ribbon commission" to keep track of all its spending, according to Miami-based UNO coordinator Leonardo Somarriba.

A House subcommittee and Senate staff members are investigating allegations that some of the $27 million in nonlethal aid provided last year was illegally diverted to offshore bank accounts, the Honduran armed forces and obscure individuals, and that some contras have been linked to drug trafficking.

The State Department provided a detailed rebuttal of the charges to undecided members of Congress on Wednesday as part of its lobbying campaign for the aid package, and President Reagan promised in a television speech that no corruption or human rights abuses would be tolerated.

The third UNO leader, Arturo Cruz, said yesterday the group welcomes the investigations. "What is at stake here is the honor of the Nicaraguan resistance, and it is too dear to us to deal with it lightly," he said.