SAN SALVADOR, JAN. 4 -- Leftist rebels, while stepping up attacks against El Salvador's battered economy, have outlined a new military offensive to overthrow the U.S.-backed government.

In a document allegedly written for combatants and given to reporters by rebels, the insurgents said their offensive would succeed "if we are capable of bringing the masses into actions of generalized violence and dividing the military."

The document made no mention of a negotiated settlement in the conflict, which has claimed about 62,000 lives. Rather, it concentrated on how the insurgents had moved back to the capital and how conditions were more favorable now than in 1981 when the rebels launched a "final offensive" that failed.

The rebels, an alliance of five Marxist-led armies grouped in the Farabundo Marti National Liberation Front (FMLN), stepped up their attacks against the nation's electrical grid, blacking out seven of the nation's 14 provinces today by blowing up more than a dozen power lines, according to officials.

On Saturday, the insurgents attacked a large coffee plantation in central San Vicente province, burning almost 900 tons of coffee, three trucks and several buildings. Damage was estimated at $2 million.

The U.S. Embassy estimates that the insurgents have caused almost $2 billion in economic damages in the past eight years, including lost and destroyed production and damage to the country's infrastructure.

El Salvador has received almost $3 billion in U.S. economic and military aid since the war began in 1979. The United States provides about 55 military advisers.

According to the rebels, the government of President Jose Napoleon Duarte is too divided internally to cope.

Col. Mauricio Ernesto Vargas, head of the Army's operations, said of the rebels' publicized plans: "Militarily, I believe there is little they can do, other than carry out actions of terror like blowing up power lines. But there are certain factors, such as our economic crisis leading to our social crisis, which could lead to a situation of increased violence.

"If there is polarization in this year's election {for the National Assembly} that produces frustration, or the economy gets much worse, they may be able to take advantage of that. But they cannot win."

"The FMLN is completely engaged in the vast task of organizing this offensive, taking into account all plans of action," the rebels' document said.

While most military analysts agree the FMLN is one of the best-organized guerrilla forces in Latin America, they say the military's overwhelming firepower and manpower are sufficient to thwart the rebels.

The U.S. Embassy and the military high command say publicly that the war is winding down, but some top officers have said informally that the conflict could drag on indefinitely, as each side adjusts to the other's changes in strategy.

In the past two years, the insurgents, responding to the effective use of helicopters by the military, have broken down into smaller units, spread out their forces and concentrated their attacks on economic rather than military targets.

However, as their attack last March on the military barracks of El Paraiso demonstrated, they still have the capability to launch major military actions. Close to 100 soldiers were killed in the attack, along with an American military adviser.

The rebels also have tried to radicalize labor unions and other organizations in the capital. In recent weeks, they have stepped up their urban activities, killing five policemen and blowing up telephone exchanges and light poles.

Many diplomats and analysts say the moves have been counterproductive, frightening people away from the rebels instead of driving them to the insurgents.

"It was too much, too fast," said Ignacio Ellacuria, rector of the Jesuit-run University of Central America, who has maintained close contact with the insurgents.

While the Army has grown from 12,000 in 1980 to 54,000 now, the analysts estimate that rebel forces have shrunk from close to 12,000 to about 6,000.

In 1987, the military launched a series of sustained campaigns and stayed for several months in areas the rebels normally control. But the rebels have demonstrated an ability to return to those areas as soon as the Army leaves and maintain a political and military structure there.

"We are seeing growing frustration, especially among younger officers, who now feel this may never end," said one military source.