U.S. military officials in Central America, reassessing Nicaraguan support for El Salvador's rebels, say that foreign arms shipments to the insurgents are probably not as extensive or as vital to the outcome of the Salvadoran war as they once appeared to be.

Diplomats and other observers say the guerrillas have captured large amounts of weapons from the Salvadoran military. According to one U.S. diplomat, the guerrillas have probably captured enough arms from the Salvadoran armed forces during the past four months "to sustain their needs" if some supplemental ammunition is brought in from the outside.

Despite the insistence of administration officials in Washington that Nicaragua is continuing to provide a significant amount of weapons to the Salvadoran rebels, diplomats and military officials in Central America say that for more than a year there has been very little solid evidence of material support for the Salvadorans originating in Nicaragua.

"It is very possible Nicaragua is not feeding anything but peanuts into El Salvador," said one U.S. official concerned with investigating the traffic.

Another informed military officer said, "I never have thought Nicaraguan arms supplies are critical." He estimated that about "20 to 40 percent" of the guerrillas' arms may come from Nicaragua "at peak times."

"That doesn't make much difference," said the officer, except that "psychologically it's nice to know a sponsor's still there."

Administration officials in Washington, however, have continued to stress the importance of outside arms supplies to the Salvadoran guerrillas. Secretary of State George P. Shultz said at a hearing of the House Foreign Affairs Committee last week that arms "from the Soviet Union to Cuba, Nicaragua and these insurgents" fuel the war in El Salvador. A State Department official said the Salvadoran rebels "couldn't sustain the type of fighting they're engaged in now, without outside arms and coordination from Nicaragua."

In December, Central Intelligence Agency Director William Casey sought to calm congressional concern over reports of covert U.S. efforts to overthrow the Sandinista rulers of Nicaragua. In closed-door briefings, he told congressional intelligence committees that interdicting the arms supply to the Salvadoran guerrillas was the administration's chief goal in supporting covert operations in the region.

A growing concern among some U.S. officials in Central America is that placing too much emphasis on foreign arms supplies as the source of El Salvador's current problems allows the military here to sidestep the serious social, economic and political problems in which the conflict is rooted.

Col. Mario Enrique Acevedo, commander of Morazan province where some of the guerrillas' strongest units continue to operate, recently told a group of journalists that he gets frequent local reports of clandestine arms supply flights. He said he had firm intelligence of a Soviet military officer who spoke fluent Spanish landing at a guerrilla camp and giving orders to the insurgents. The story was a typical one, but also typically, his intelligence did not include a date for this vividly described scene.

Such stories circulated by and among some Salvadoran officers are related to the feeling that some of them have that they are really fighting Washington's East-West war and that Washington should be doing more of the job itself.

Perhaps if El Salvador had oil, said Acevedo, using an argument common among his peers, there would be no problem about U.S. support. "They would send us one of those brigades that doesn't let anybody get by."

The rebels deny receiving any important amount of weapons from sympathetic governments. They say they buy their arms on the international black market, from corrupt or sympathetic members of the government's armed forces or capture them in combat. A recent guerrilla statement said they have seized more than 1,200 "weapons of war" from government forces since October.

U.S. officials say this figure is probably exaggerated but acknowledge that "large" quantities of arms have been taken by the insurgents. These include everything from pistols to assault rifles to 120-mm mortars.

The issue of Nicaraguan support for the Salvadoran rebels has been a major obstacle in relations between Washington and the revolutionary Sandinista government in Managua. Efforts to interdict arms shipments from Nicaragua or to pressure the government there into cutting off all support for the Salvadoran insurgents have colored U.S. policy throughout Central America.

Few U.S. military officials in the region question Washington's overall policy toward Nicaragua, since they view Managua's internal arms buildup and the Sandinistas' ideology as a menace to Central American peace.

The Sandinistas say openly that they provide moral support and "office space" to the Salvadoran guerrillas. Nicaraguan Commander Bayardo Arce, in charge of relations with other revolutionary movements, said in an interview last year that the Sandinistas had shown the Salvadoran rebels the clandestine connections and networks they used to get arms for their own 1979 insurrection. Cuban officials have said they sent large quantities of arms to Salvadoran revolutionaries in late 1980.

Early in the Reagan administration, then-secretary of state Alexander M. Haig Jr. said Nicaragua was the main channel through which Soviet and Cuban arms were sent to the Salvadoran guerrillas. During the past two years, the threat of a guerrilla victory in El Salvador has been used to justify a large increase in military aid to Honduras.

Last fall, high-level officials said most land and sea shipments of arms through Honduras and Nicaragua had been stopped but that air shipments had greatly increased.

Some U.S. officials in Central America say that the Honduran Army, aided by U.S. advisers, has virtually shut down the guerrillas' overland supply routes from Nicaragua to El Salvador. Some U.S. diplomats in Honduras and El Salvador maintain that the slack has been picked up by increased air and sea deliveries of arms to the insurgents.

A State Department official said the administration believes the supply of arms from Nicaragua is continuing at a "relatively high level" and includes heavy weapons such as mortars. He said the arms supply is carried out "far more by air now than in the past."

But despite administration assertions of success in interdicting arms shipped by land, and more recent charges that shipments are continuing, primarily by air, not a single major shipment of arms has been captured in or near El Salvador since a Costa Rican pilot was caught in 1981. The one major smuggling network that was uncovered last year also had its base in Costa Rica.

One intelligence officer who has interviewed numerous residents along presumed nocturnal flight paths between Nicaragua and El Salvador cautioned that for some people, attuned to thinking in such terms, "every plane that flies is a 'gunrunner.' "

The most conspicuous example of this type of thinking, according to U.S. officials, was the sighting of what was thought to be a Soviet-built helicopter by a group of conservative U.S. lobbyists and polticians when they visited El Salvador last month in a delegation led by Rep. Philip M. Crane (R-Ill.)

On a stop at the Salvadoran port of La Union on the Gulf of Fonseca opposite Nicaragua, members of the group thought they saw a twin-rotor "Russian helicopter" leaving what appeared to be Nicaraguan airspace and heading toward guerrilla-dominated areas of northern Morazan province in El Salvador.

According to U.S. officials who investigated the incident, the aircraft was "almost certainly" a U.S. "Chinook" helicopter on its way from Panama to Honduras in preparation for joint military maneuvers there at the beginning of this month.

One element in Washington's appraisal of the arms-supply problem is electronic and photographic intelligence to which most U.S. officials on the isthmus say they have had no access. Although the administration last year publicly presented air photographs of what it said were internal Nicaraguan military bases, heavily fortified and modeled along Cuban lines, a subsequently scheduled presentation of similar information on arms shipments was never held. U.S. diplomats say that the information has not even been shown to the Salvadorans to aid them in catching alleged smugglers.