Top-ranking Salvadoran guerrilla commanders have vowed to extend their fight throughout this small country and denied that the U.S.-backed government was winning the five-year-old civil war.

In a rare news conference with U.S. reporters Friday, in guerrilla-controlled territory, the commanders reaffirmed their intention to attack U.S. military personnel stationed in El Salvador following the slaying of four Marine embassy guards June 19 at a sidewalk restaurant in San Salvador, the capital. Attacks such as the one on the marines "raise morale for our people, without doubt, and demoralize our enemy," said Joaquin Villalobos, considered "first among equals" in the five-man General Command of the Farabundo Marti National Liberation Front. The guerrillas claimed responsibility for killing the marines shortly after the attack.

Another senior commander, Jorge Shafik Handal, said, "We consider as military objectives all those who are engaged in this war of counterinsurgency against us."

The commanders tended to steer away from discussing the restaurant attack, but they said they planned to step up activities, including killings in the capital, and directly challenged the United States.

"What does the administration think it will do when it only has a year of its term left and El Salvador is not resolved? What idea is it going to propose? Send the troops?" Villalobos asked.

The guerrilla officials held the news conference for six correspondents who were invited to this bombed-out mountain village in northeastern El Salvador that serves as the informal capital of rebel-held territory.

Villalobos, leader of the People's Revolutionary Army, which is one of the two main guerrilla groups in the five-faction FMLN, rarely grants interviews, and no U.S. journalist is known to have seen him before.

Handal, 56, is commander of the Armed Forces of Liberation, a smaller guerrilla force, and an official of the Salvadoran Communist Party who has links to countries such as Cuba and the Soviet Union. High-ranking leaders, but not the top commanders, also were present representing the other three guerrilla forces.

The FMLN staged the news conference to try to show the world, and particularly the U.S. public, that it is determined to continue its struggle despite the Salvadoran armed forces' improved military performance in the last 18 months, and despite the recent electoral triumphs of President Jose Napoleon Duarte and his Christian Democratic Party, rebel officials said.

The commanders held out little hope for a negotiated settlement, saying that time was on their side in what they called a "war of attrition." They blamed Duarte for lack of progress in peace talks, and categorically rejected the president's demand that they stop fighting and compete for power in elections as provided for by the country's new constitution.

"There is no condition under which we would lay down our arms, because we are not willing ever to lay them down," Villalobos said. Handal added: "How could it occur to anybody that, in a country at war, it's necessary to wage the war constitutionally? We really have no commitment to this constitution."

The commanders blamed the United States for their failure to achieve victory, saying the Salvadoran armed forces recovered from setbacks in 1982 and 1983 only because of U.S. aid.

"We believe that, except for the escalated process of U.S. intervention, we would have won the war," Villalobos said. He said that halting U.S. support for the government was the guerrilla front's "fundamental" condition for reaching a negotiated settlement.

The Salvadoran armed forces, backed by hundreds of millions of dollars of U.S. assistance, have regained the military initiative since the rebels' last sustained offensive in late 1983. The Army has pushed the guerrillas back into county-sized enclaves here and in the southeastern and northern parts of the country. It has forced the rebels to travel and fight in small units of several dozen, while in 1983 they moved in groups of hundreds, according to rebel, Salvadoran Army and U.S. officials.

But the commanders argued yesterday that the war merely had entered a new stage, and said that they were stepping up small-scale infiltration around the capital and in southern and western parts of the country where they have not been very active in the past. Rather than seeking to expand their territory as before, the guerrillas are concentrating on killing Army soldiers and sabotaging the economy to wear down the government, they said.

"Our thinking is that in one year, we will be waging the war in all of the country's territory, in all the highways, in all the cities," Villalobos said. "The Reagan administration can send all the rifles it wants, but it cannot replace the Army's casualties."

The commanders asserted that they could continue their war indefinitely because the Army was unable to catch them despite regular sweeps through the mountainous and swampy terrain where they have their bases.

The press conference was conducted here just 11 days after a helicopter-borne Army battalion had left the area following a 10-day operation. At the time of the news conference, Army troops were stationed as close as nine miles south of here on the southern banks of the Torola River, which has served for three years as the border between guerrilla-dominated and Army-dominated territory in this region.

In the 90-minute news conference, held on the covered patio of a house on Perquin's main square, the commanders also made the following points:

*Without admitting that the guerrilla front received arms from Nicaragua, they strongly defended their right to seek assistance wherever they wished. They ridiculed the U.S. administration's position that the Salvadoran insurgency was part of a Soviet-designed plan to export revolution, saying instead that the front had sprung from opposition to the unequal distribution of wealth in this country and from the armed forces' brutality in repressing popular movements.

"The first arms to enter [from abroad] were from the United States," Handal said, referring to the sending of $5 million of military assistance here by the Carter administration in 1980. "If we receive arms, the proportion would have to be something like 1,000 to 1" between arms supplied by Washington to the government and those supplied by Nicaragua and other left-wing countries to the FMLN, he said

The commanders seemed to be defensive about several recent guerrilla actions against civilians.

They admitted, for instance, that they had made an "error" last year when Villalobos' force kidnaped several hundred youths in the eastern part of the country and tried to force them to join their ranks. The press-ganging, which began in April and lasted about three months, hurt the guerrillas' relations with the civilian population and drew criticism abroad. Most of the youths were released later.

In addition, Villalobos defended his force's kidnaping of 17 mayors this spring, and the killing of one or two of them. He added that the kidnapings had ended "because there aren't any more mayors to kidnap."

*On another issue involving treatment of civilians in the war, the commanders denied that the urban commando unit that killed the four U.S. marines last month was responsible for slaying nine civilians, including two U.S. businessmen, who died in the attack. Handal endorsed the position, taken by the unit that staged the assault, that unidentified persons at the restaurant had fired back at the attackers and killed the civilians.

"It's not true that this was an operation to massacre, to kill. It was an operation directed against military objectives," Handal said. The consensus of witnesses and investigators, however, is that the guerrillas killed the civilians even though there were reports that one or two diners may have fired back.

On a related point, Handal said that the guerrilla front was not concerned about criticisms of the restaurant attack by members of the Democratic Revolutionary Front, a group of exiled, leftist politicians who are allied with the FMLN.

"We don't have an absolute identity [with the civilian front], either in our form of fighting or in our political positions," Handal said. "We don't consider this [criticism] a rupture or anything serious."

The other guerrilla leaders at the conference were Facundo Guardado of the Popular Liberation Forces, Leo Cabral of the Armed Forces of National Resistance and Miguel Mendoza of the Central American Revolutionary Workers Party.

The youthful-looking Villalobos was a founder of the People's Revolutionary Army and is considered the FMLN's most prominent hard-line leader and its chief military strategist. He was an urban guerrilla leader in the late 1970s and a U.S. Embassy document has suggested that he may have planned attacks on the embassy in San Salvador during 1980. He moved to the countryside later and claimed, with other commanders, to have established control of the area around Perquin in August 1981.

The journalists -- five from U.S. media and one from a Mexican television network -- drove here Thursday with a seven-member delegation of representatives of liberal Hispanic-American organizations from California. The delegation arranged the reporters' visit.

Rep. George Brown (D-Calif.) has accompanied the delegation on several of its stops, and he contacted senior Army officers here to tell them that the delegation and journalists were making the trip and should not be attacked, according to members of the delegation. Brown did not visit Perquin after being advised against it by the State Department, they said. The group, traveling in four vehicles, was waved through a series of Army roadblocks without incident.