CENTRAL AMERICA : New Savagery Dims Hopes for an Early Truce in El Salvador : As another round of peace talks begins, both the government and rebels have stepped up their tactics of violence.

El Salvador’s long, exhausting civil war is entering an even nastier stage with leftist guerrillas reverting to terrorism and government troops intensifying a nationwide offensive aimed at weakening rebel territorial claims in advance of a possible cease-fire.

The stepped-up violence is going on even as negotiators gathered Friday in Caracas, Venezuela, to begin another round in what has become an agonizing effort to sign a cease-fire in the war that already has claimed an estimated 70,000 lives.

Although both the government and the leaders of the guerrilla Farabundo Marti National Liberation Front predict success at the U.N.-sponsored negotiations, each side appears willing to test the limit of the other’s tolerance to accept punishment. That testing leads most observers to doubt that a cease-fire can be reached for months at least.

Government officials say it was the rebels who initiated the current bloodletting with wide attacks on the nation’s electrical system--raids aimed at pressuring President Alfredo Cristiani into concessions.

“The FMLN is positioning itself by continuous sabotage, instigation of strikes, land invasions by peasants,” said Ernesto Autshul, one of Cristiani’s key aides. “It is part of a strategy to get attention and keep the rank and file together.”

While that certainly is part of the rebel tactic, the more immediate cause of the current wave of death and destruction was the decision to begin a three-month government offensive, which has sent troops and air power into areas long under FMLN control.

This offensive’s “true intention is to push back the guerrillas” so they will have smaller zones of control under any cease-fire, a European diplomat said, speaking of an anticipated agreement that will give both sides secure areas while a permanent peace is arranged.

As it stands, the rebels control or operate freely in segments totaling about a third of El Salvador. But the government offensive is challenging that hold with troops suddenly appearing in areas where they haven’t fought in seven years.

The response of the guerrillas was first to attack the national electrical system to force the army to drop its offensive and defend the power facilities. The military reply was to call up reserves and keep up the offensive, foreign military experts said. The rebels then countered by widening their sabotage so much that more than half of the country is without power at least part of every day.

But the FMLN also has resumed terrorist tactics that they had pledged to abandon. There are increasing threats to kidnap and kill judges and mayors who cooperate with the government--even if their only action, say, is to agree to accept water projects.

The worst incident occurred Wednesday night when the FMLN fired nine rocket-propelled mortar shells at the headquarters of the army’s 1st Brigade in a crowded residential neighborhood in the capital. Only one of the wildly inaccurate shells hit the huge installation. The others fell into houses, streets and cars, killing two women and wounding other civilians. “This is terrorism pure and simple,” said a Latin American diplomat normally tolerant of the FMLN. “It won’t do them any good.”

While the rebels are taking heat for their actions, however, the army has hidden its activity in the countryside and refused to allow foreign reporters access, leading to fears of government abuses, human rights workers say.

“It makes you wonder what they are doing,” said one. “In the past, when they operated in secret, it was a sure sign of brutality and attacks on civilians. We can’t say that for sure because we can’t investigate, but it worries me.”