Nicaragua, Cuba and other leftist countries have played the leading roles in arming and training El Salvador's left-wing guerrillas since 1980 but gradually have curbed their support since 1983, according to information gleaned from a recent defector from the rebels, a U.S. study of captured weapons and a stash of captured rebel documents.

U.S. pressure has led Nicaragua's Sandinista government to withdraw some of its backing for the Salvadoran rebels on several occasions, both by suspending ammunition shipments and by restricting the activities in Managua of the rebels' Farabundo Marti National Liberation Front, according to these sources.

For example, captured rebel notes and correspondence indicate that Nicaragua cut back assistance following the U.S. invasion of Grenada in October 1983 in an action that drew strong protests from the Salvadoran guerrillas.

It was unclear from the documents and other information how much aid Nicaragua has been contributing in recent months, but ammunition shipments appear to have dropped substantially. The defector, a former political and military commander, said he was aware of only two deliveries this year.

The newly available sources offer a broad portrayal of the history of external support for the Salvadoran rebel front, known by the Spanish initials FMLN. While some have questioned the reliability of the defector's account and of the documents, the new data tended to confirm descriptions provided for the past two years by U.S. and Salvadoran officials.

"The embassy's position is, damn it, we told you so," a senior U.S. official said.

The government and the U.S. Embassy said the documents were seized April 18 with prominent rebel commander Nidia Diaz, who was a member of the guerrilla delegation at the peace talks in the town of La Palma last October. Salvadoran authorities have refused her requests to meet with reporters since her capture.

The FMLN has charged that the documents are forgeries.

According to the portrayal of support for the guerrillas gleaned from the new sources, leftist nations initially contributed 6,000 to 7,000 automatic rifles plus mortars and grenade launchers from 1980 to early 1983. These arms are described as having arrived from Nicaragua by clandestine means, mostly in small planes or overland through Honduras.

Cuba, the Soviet Union, Vietnam, Bulgaria and East Germany have trained a steady stream of guerrilla leaders in military and political work, by this collective account. The FMLN's general command met regularly in Managua in the early 1980s.

Since roughly two years ago, however, foreign military assistance has consisted primarily of ammunition and explosives, and it increasingly has arrived by sea.

"Most people agree that the big arms imports stopped in 1982," the senior U.S. official said.

Shipments appear to have dropped off for three reasons: because they were not needed, because of U.S. pressure on Nicaragua and because of the Honduran armed forces' breakup of much of the FMLN's clandestine support network in Honduras during 1982 and 1983.

The Sandinistas also began to pull back the welcome mat in Nicaragua in mid-1983, according to the defector. At that time, the FMLN's general command was forced to transfer its meetings to rebel-dominated territory in El Salvador after Nicaragua was embarrassed by the murder of a senior Salvadoran rebel commander in Managua in a factional dispute, he said.

The issue of Nicaraguan and other outside aid for the Salvadoran rebels has been a central feature of the U.S. debate over Central America. The administration repeatedly has asserted that Nicaragua was "exporting revolution," and it used this charge specifically to justify organization and financing of the rebel force now fighting to overthrow the revolutionary Sandinista government in Managua.

Critics of U.S. policy have said the administration lacked adequate proof of a steady, substantial flow of military aid to the guerrillas since their failed "final offensive" in January 1981. Despite a high-priority effort, no arms shipment in progress from Nicaragua has been intercepted.

Salvadoran rebel leaders and Nicaraguan officials have offered cautious and sometimes conflicting responses to the U.S. charges. The Salvadoran rebels have admitted, for instance, that they smuggle weapons and ammunition via Nicaragua but have said they obtained the arms on the international market and not from the Sandinista government. Nicaragua has acknowledged giving diplomatic and moral support to the guerrillas while denying that it was shipping ammunition.

The defector -- whose real name is Napoleon Romero, but who often is known by his nom de guerre, Miguel Castellanos -- said that about 70 percent of the FMLN's automatic rifles came from abroad and that the rest were captured from the Salvadoran armed forces. While some of the foreign-supplied weapons were purchased on the international market, he said, most were supplied by friendly governments.

Romero, 35, was commander of the San Salvador front for the Popular Liberation Forces, one of the two largest of the five guerrilla forces in the FMLN, until his defection in early April. He said in a 90-minute interview that he had become disillusioned with the revolutionary movement during the past year because of its violence and lack of accomplishments.

Romero was held by the military for several weeks before being made available to reporters. He is now said to be living under the protection of the military.

The guerrilla organization has charged that Romero was tortured while in custody and is now lying.

As a defector, Romero has an interest in portraying the FMLN in a bad light, but his articulate responses seemed frank during the interview. It was conducted in an office of the armed forces' press committee. A Salvadoran major was present for only brief portions of the interview.

Romero's description of the source of the weapons was bolstered by a U.S. military intelligence survey of serial numbers of U.S.-made M16 automatic rifles captured from the guerrillas. The survey's results, made available by U.S. officials, showed that just under 25 percent of the rifles originally were provided to the Salvadoran Army and thus presumably had been captured by the rebels. Of the remaining rifles, the bulk were said to have been left in Vietnam by evacuating U.S. troops in 1975.

Romero said he believed Cuba was responsible for coordinating much of the international support for the Salvadoran guerrillas.

"Nicaragua is just the bridge for everything coming from Cuba," he said.

The defector also noted several areas in which leftist countries have not been involved much with the FMLN. For instance, he said that he believed no Cuban or Nicaraguan official advisers or instructors had come to El Salvador to oversee the guerrillas' fight directly.

In addition, while each of the five factions in the FMLN has its own radio transmitter for direct communications with its representative in Managua, Romero said, the insurgency is not "directed" from Nicaragua on a day-to-day basis as the U.S. government has suggested. FMLN commanders in the field plot their own strategy and tactics, although they often solicit advice from Nicaragua and Cuba, he said.

The captured documents that have been made public so far show the FMLN's dependence on Nicaraguan support mainly by revealing the level of concern on the part of the guerrilla organization in late 1983, when the Sandinistas were pulling back their support. Minutes of meetings, briefing papers and letters show the Salvadoran guerrilla leadership pressing hard for continued backing.

A two-page list of names of FMLN leaders and foreign training courses that they had undergone or were scheduled to undergo also showed the involvement of a wide range of Soviet Bloc countries in seeking to build a cadre of Salvadoran revolutionary leaders.