Leftist rebels Wednesday attacked army positions in the north and stepped up their campaign against Sunday's elections by threatening to kill election workers.

At least three soldiers and six guerrillas were killed, police and army sources said.Guerrillas of the Farabundo Marti National Liberation Front struck several garrisons and posts in the northernmost province of Chalatenango, army and rebel reports said.

The rebels' clandestine Radio Venceremos said 13 soldiers were killed or wounded in the town of Tejutla, 30 miles north of San Salvador.

The radio said guerrillas also hit the garrison in the provincial capital of Chalatenango, 28 miles northeast of the capital, and army or civil defense posts in three other towns.

The radio did not immediately provide casualty figures.

Col. Ciro Lopez Roque, commander of the 4th infantry brigade, said in a radio interview that the army had no precise figures but that "there were quite a few casualties."

The armed forces press committee said three soldiers died and three were wounded in the fighting.

A police spokesman, speaking on condition of anonymity, reached by telephone in Chalatenango said the bodies of six guerrillas had been recovered. The rebels also broadened an implicit threat made earlier this week.

"The polling places will be sabotaged and those persons who lend themselves to this electoral game will later be killed, because participation indicates agreement with escalation of the war," said a rebel statement sent to radio stations.

The leftist Farbundo Marti National Liberation Front said earlier this week that election officials would be considered part of the counterinsurgency campaign. The threats were similar to those that have been made against mayors.

Nine mayors have been assassinated. More than half the country's 262 mayors have resigned because of the threats.

"The guerrillas' opposition to the elections confirms the belief that what they want is to take power by force," said President Jose Napoleon Duarte in a Tuesday television interview.

Sunday's election presents Salvadorans with their first opportunity to vote out an incumbent civilian administration and polls indicate they may do just that. The rightist Republican Nationalist Alliance, led by Alfredo Cristiani, is favored to beat the Christian Democrats.