Secretary of State George P. Shultz said here today that Nicaraguan fears of a U.S. invasion were "self-induced and based on nothing" but that the United States intended to "work in every way that we can" to cast Nicaragua's "aggression and subversive influence out of our hemisphere."

In a press conference following the opening of the Organization of American States' general assembly, Shultz said Nicaragua's Sandinista leaders appeared to be warning of an invasion "to whip up their own population."

"I don't know why they are doing this," he said. "It is certainly a problem in the region that they continue to import heavy armaments. As far as invasion fears, they seem to be self-inflicted wounds on the part of Nicaragua."

Shultz said that to counteract what he described as the threat of Soviet arms shipments, the Reagan administration has "in the first place to help our friends put themselves in the capacity to resist the aggression that comes from these arms. And we have been doing that."

"What we are trying to do about it is to discourage these shipments, to make it more and more difficult for them to be used against the neighbors of Nicaragua and to work in every way that we can to cast this aggression and subversive influence out of our hemisphere," he said.

The secretary of state's remarks followed an address to the OAS assembly in which he reiterated U.S. concern over a "dangerous military imbalance" in Central America. He also called on Latin American nations to initiate "a greater multilateral effort" to combat terrorism and take "immediate action" against international drug trafficking.

Shultz, who met today with foreign ministers of U.S. allies in Central America as well as Contadora group members Mexico and Colombia, stressed the U.S. position that "credible verification and control mechanisms" must be included in any peace settlement for Central America.

"Good words will not guarantee that armed opposition groups will be integrated into a genuinely democratic political system," he said in the address. "Promises will not be enough to guarantee that one nation is not a military threat to another. Promises will not reduce an already dangerous military imbalance that is constantly fed from outside the hemisphere."

Shultz's speech contained no direct mention of Soviet arms shipments to Nicaragua and little direct criticism of the Sandinista government, an omission officials here said reflected deference to traditional OAS protocol rather than a moderation of the U.S. position on arms shipments.

The Reagan administration and U.S. Central American allies recently rejected a Central American peace proposal by Contadora nations Mexico, Colombia, Venezuela and Panama that included a freeze on armaments and a ban on foreign military bases and advisers in the region.

Officials said Shultz discussed changes proposed in the draft treaty in his closed meetings with the Central American and Contadora group ministers today. Honduras, Costa Rica and El Salvador have proposed amendments providing for international verification of negotiated arms reductions that would be carried out simultaneously.

Officials said that the Contadora foreign ministers plan to meet here to consider the proposed revisions but that no action on the issue was expected here this week. Both Mexican Foreign Minister Bernardo Sepulveda and Venezuelan Foreign Minister Isidro Morales Paul indicated yesterday that some aspects of the treaty changes would be unworkable.

In his address today, Shultz said that the Reagan administration has begun a new antiterrorist training and assistance program for civilian agencies of allied governments, paralleling already established military training programs.

Responding to calls by Latin governments for reform in the system of managing their foreign debts, Shultz also reiterated the U.S. view that "the initial debt crisis has been managed" and that Latin nations should seek renewed growth through "internal adjustments" and the pursuit of private foreign investment.

His remarks stood in sharp contrast to those of Brazilian President Joao Figueiredo and OAS Secretary General Joao Baena Soares, who said that present formulas for managing the debt burden were inadequate. They supported recent calls by Latin nations for negotiations with industrialized countries on reform of the international financial system.

"Despite our efforts, we have not received proposals from the creditor nations that are capable of covering the complexity of these problems and attend the greater ill of underdevelopment," said Figueiredo in a speech opening the assembly.

Officials of the 11 most indebted nations, the Cartagena group, told Shultz yesterday that a U.S. proposal to discuss debt issues within the International Monetary Fund was not sufficient.

Although the foreign ministers spoke exclusively on the debt and other economic issues yesterday, the six-day OAS assembly is unlikely to take action on the debt or regional security issues, officials said.

The most important action at the meeting, OAS officials said, may be a decision on multilateral efforts to combat the drug trade. The assembly will consider a proposal to create a special OAS fund to contribute to crop substitution and other anti-narcotics programs in member countries.

The assembly is also expected to accept a committee recommendation that the $36,000 pension of former secretary general Alejandro Orfila be suspended because of Orfila's acceptance of a salaried private sector position several months before his retirement from the OAS last year.

Baena Soares, a Brazilian diplomat who took over as secretary general this year, said a principal priority of the OAS should be "an in-depth reflection" on how to revive the organization "in an international setting that seems unfavorable and even dangerous."