The U.S. government plans soon to supply El Salvador with four rapid-fire helicopter gunships that the administration believes will expand significantly the ability of that country's armed forces to patrol roads and carry out quick-reaction airborne assaults, U.S. officials said this week.

The Hughes 500 helicopters are to be equipped with multiple-barrel guns capable of firing 5,000 to 6,000 rounds a minute, roughly double the maximum rate of fire of two C47 airplane gunships delivered to El Salvador in December.

The use of the C47 airplanes aroused controversy in Congress -- where some Democrats expressed concern that the steadily escalating air war in El Salvador increases the likelihood of civilian casualties -- and the delivery of the new Hughes helicopters appeared likely to fuel further criticism. In addition, new questions have arisen about three Hughes 500 helicopters, one equipped with a similar rapid-fire gun, already in the Salvadoran arsenal.

Congress generally has backed the administration's policy in El Salvador since the inauguration in June of civilian President Jose Napoleon Duarte, but the air war is one issue regarding El Salvador on which congressional skepticism remains high. Congressional critics say they are particularly concerned that the administration is strengthening the Salvadoran Air Force without a full debate on the types of weapons being provided or on how they are to be used.

In a letter last fall to Thomas R. Pickering, the U.S. ambassador to El Salvador, four congressional Democrats, including Edward P. Boland (Mass.), then House Intelligence Committee chairman, and Michael D. Barnes (Md.), chairman of the House Foreign Affairs subcommittee on Latin America, expressed concern about the use of the C47s and asked to be kept informed as to their use.

Several congressional staffers who have followed the issue closely said this week that the new helicopters are likely to come under similar scrutiny.

San Salvador's Roman Catholic Archbishop Arturo Rivera y Damas has criticized use of such aircraft because of the danger of causing civilian casualties with their ground-blanketing fire. The U.S. and Salvadoran governments say that the Air Force has specific instructions to hold fire if civilians could be hit.

The four new helicopters were approved as part of a supplemental appropriation last year. But at least one member of Congress has raised concerns about the other three Hughes 500 aircraft, already in El Salvador, that were not included in a list of U.S.-supplied weaponry that the State Department provided to Congress in November.

Sen. Edward M. Kennedy (D-Mass.) yesterday sent a letter to Secretary of State George P. Shultz asking how El Salvador had acquired the three aircraft, and the rapid-fire gun, and "in what situations they are used."

"The report submitted to Congress accounts only for transfers funded by the security assistance program," he wrote, "although the law clearly requires an accounting of all U.S. military equipment transferred to El Salvador."

In response to reporters' queries this week, U.S. officials said the three helicopters were not provided to El Salvador as part of a routine military aid package. Neither the State Department nor the Pentagon provided a response to queries about their origin.

Of the three Hughes helicopters already in El Salvador, the one equipped with a rapid-fire gun has been used to support airborne assaults and provide covering fire for ground troops, a military source in San Salvador said. He said one of the three was obtained in 1979 and the other two came in 1982 or 1983, although he did not know from where.

The manufacturer, Hughes Helicopters Inc., said yesterday in response to a query that it had sold nonmilitary helicopters "in El Salvador" through one of its distributors. It did not say when the sales took place, how many helicopters were involved, or who paid for them.

El Salvador's U.S.-supplied Air Force has expanded substantially since Duarte's inauguration. Its fleet of Huey helicopters, equipped with machineguns but used primarily to ferry troops, has nearly doubled to 39, including four medical evacuation helicopters. Last month, three A37 Dragonfly jets, equipped with bomb racks, were added to the six already there.

The new helicopter gunships are "in the pipeline" and could arrive within the next three months, a U.S. Embassy spokesman in San Salvador said Tuesday.

According to U.S. officials and military sources, the craft will be used to protect convoys, drive guerrillas off roads that they frequently cut, and provide cover for Salvadoran troops rushed by helicopter to respond to rebel assaults. U.S. military personnel in El Salvador long have favored building up the capability to stage helicopter assaults on guerrilla concentrations.