U.S. government and private experts said yesterday that recent improvements in Nicaragua's armed forces appear largely defensive or aimed at defeating indigenous CIA-backed rebels, not at invading neighboring countries.

Defense Secretary Caspar W. Weinberger said on Sunday that Nicaragua is receiving a "tremendously increased flow of offensive weaponry" that has "the effect of intimidating their neighbors." Speaking on NBC's "Meet the Press," Weinberger said he was referring to "ammunition, heavy weaponry, shells, things that will improve their capabilities in the form of attack helicopters."

Other officials, speaking on condition that they not be identified, said it is often difficult to distinguish between "offensive" and "defensive" weapons. But they said that most of Nicaragua's recent imports are either clearly defensive, such as antiaircraft guns and missiles, or especially useful for antiguerrilla operations, such as the recently acquired Hind helicopter gunships.

A clearly offensive element is Nicaragua's force of 60 or more Soviet T54 tanks, which would not be useful in the mountains and jungles where the "contra" rebels usually fight. But the tanks also would not be useful for invading other countries across most of Nicaragua's rough terrain, the experts said.

"There clearly is an upgrading of their weaponry," one official said. "Whether it is offensive weaponry depends on your world view."

Asked whether the United States would con-sider naval action to keep weapons from Nicaragua, Weinberger would say only that "the United States is prepared for a number of contingencies that may have to be taken." But a senior Pentagon official dismissed as "ridiculous" suggestions that the United States is considering a naval quarantine.

"There are options from A to Z," the official said. "That doesn't mean they're being taken seriously."

Arms shipments to Nicaragua attracted attention last week when administration officials speculated that one cargo might include advanced Soviet fighter jets. When intelligence reports did not confirm those suspicions, the officials shifted their emphasis to what they described as a dangerous overall arms buildup by Nicaragua's Sandinista leaders.

"The fact is that the Soviets are supplying a great deal of heavy offensive arms to Nicaragua, we think in an attempt to intimidate the Nicaraguan neighbors, who are the ones who are obviously most concerned about this," Weinberger said.

A knowledgeable observer in Managua, however, said that he believes the new arms reflect Nicaraguan concern about the CIA-backed contra rebel force, which now numbers 10,000 or more, and not an effort to intimidate other countries or challenge the United States, according to Washington Post correspondent Robert J. McCartney. This observer said that the contras have been increasingly successful during the past six months, despite congressional refusal to continue funding the force.

A leader of the contra forces, Adolfo Calero, told the Associated Press yesterday that several recently imported Mi24 Hind helicopters are more dangerous for the contras than MiG fighters would be. Calero said that the Hinds, which the Soviet Union has used in Afghanistan, are capable of "saturation attacks" against large areas and "can obliterate entire villages."

"The Sandinistas could polish us off between now and February," he said. Congress has suspended funding for the contras until at least February, and the administration is expected to ask that funding be resumed.

Honduras and El Salvador also have been upgrading their armed forces in recent years, chiefly through rapidly growing U.S. military aid programs. Administration officials said that U.S. aid to El Salvador is aimed at indigenous leftist rebels and that aid to Honduras is chiefly a response to perceived threats from Nicaragua.

Comparisons between the armed forces of Nicaragua and neighboring Honduras are difficult because Nicaraguan officials refuse to discuss their weaponry. U.S. estimates of Nicaraguan strength tend to be greater than estimates by international institutes, and at times published U.S. estimates even differ with each other.

Most experts and officials agree that the Honduran air force is larger and more capable than Nicaragua's, with 12 French Super Mystere B2 planes upgraded and modernized by Israel. Those subsonic jet fighters "are clearly a step ahead" of anything in the Nicaraguan inventory, which has mostly Korean War-vintage planes and has "nothing remotely resembling a jet fighter," according to Georges Fauriol of the Center for Strategic and International Studies at Georgetown University.

If Nicaragua imports MiG21s, as officials there have said they want to do, its air force immediately would move "a step ahead," Fauriol said, since MiG21s are supersonic. The Nicaraguans have built an airfield capable of handling such jets and have sent pilots to Bulgaria for jet fighter training, according to U.S. officials.

Nicaragua, on the other hand, has a larger force of tanks and armored personnel carriers. In an official background paper released in July, the administration said Nicaragua owned 120 tanks and 120 other armored vehicles; a chart distributed by the Pentagon last week estimated the force at 150 tanks and 200 armored vehicles.

The International Institute of Strategic Studies, which publishes an authoritative world arms catalogue, recently estimated that Nicaragua has about 70 tanks and 120 other armored vehicles.

According to the institute, Honduras has only 16 light tanks. The Washington Post recently reported that Honduras also recently purchased 72 armored personnel carriers.

Costa Rica, which neighbors Nicaragua on the other side, has no tanks and virtually no armed forces.

Nicaragua also is the only country in the region with antiaircraft missiles. U.S. officials said that Nicaragua recently received SA3 and SA8 Soviet antiaircraft missiles, added to the shoulder-fired SA7s already in their inventory.

Fauriol said such missiles could be considered offensive "if deployed defensively during offensive operations," to protect advancing tanks, for example. But officials said the missiles are likely to be used, at least at first, to protect Nicaraguan air bases and other installations from contra attacks and from what the Nicaraguans say is the threat of U.S. attack.

The missiles could also be useful in fighting the contras, officials said, because most contra supplies are air-dropped from DC3s taking off from Honduras.

U.S. officials said they believe it is unlikely that Nicaragua would take military action against Honduras or Costa Rica, despite the presence of contras in both countries, because of the likelihood of a U.S. response. The U.S. military has forces in Panama, Honduras and within striking range from Puerto Rico and the continental United States.

But Weinberger and other officials said that a militarily strong Nicaragua can increase its diplomatic leverage even without taking action.

"I think the weapons are offensive in a diplomatic sense," Fauriol explained. "It's out of proportion with regional requirements."