El Salvador's left-wing rebel alliance announced today that it would respect a truce for a total of six days during the Christmas and New Year's holidays, a pledge that unilaterally widened an earlier agreement with the government.

The insurgents also released 43 captured Army soldiers at a meeting inside El Salvador with Roman Catholic leaders and international Red Cross officials, a rebel alliance official said here.

The alliance, called the Farabundo Marti National Liberation Front-Democratic Revolutionary Front, pledged "a total truce" from midnight Dec. 23 through midnight Dec. 26, and likewise from Dec. 31 through Jan. 2, according to a communique released here by officials of the alliance's political-diplomatic commission.

The rebels' pledge appeared to bolster efforts, discussed in recent negotiations with the government, to "humanize" the war. The insurgents and the government agreed late last month, in the second round of this fall's unprecedented top-level talks, to permit free circulation of traffic on the nation's highways between Dec. 22 and Jan. 3.

The cease-fire, if respected, would be the second in five years of civil war. The first such truce, for two days, was for Pope John Paul II's visit to El Salvador in 1983.

"Our forces will suspend all offensive military action, to permit government soldiers to enjoy leave and meet with their families and friends, and so that all the people can enjoy the atmosphere created by this decision of our front," said the 2 1/2-page communique, signed by the General Command of the Farabundo Marti front, which is the alliance's military wing, and by the Executive Committee of the Democratic Revolutionary Front, or civilian wing.

"Our military activity in the designated period will be limited to defense of our troops, the civilian population, or territories under our control [that] are attacked by the government Army," the communique said. It called the action "a demonstration of willingness to advance, creating conditions to favor solution of the conflict by means of dialogue and negotiation."

Milton Mendez, military chief in one of the guerrillas' four military zones inside El Salvador, headed the rebels who delivered the prisoners of war to a delegation including Salvadoran Archbishop Arturo Rivera y Damas, a representative of the rebels, said.

The meeting took place in La Joya in central San Vicente province, the official said. The guerrillas claim to have released more than 2,000 prisoners during the war.

The released soldiers had been captured Dec. 1 in El Salto in La Paz province in a raid described by some U.S. officials as one of several setbacks this fall for the government. Others were last month's large-scale guerrilla attack on Suchitoto, near San Salvador, and the death in an October helicopter crash of the Army's most powerful field commander, Lt. Col. Domingo Monterrosa.

A high-ranking member of the rebels' civilian wing said the recent peace talks were not expected to result in the left's participation in spring legislative elections. He charged that the Christian Democratic Party "doesn't want us to run because we would take votes away from it in the election. It wants to win a majority in the assembly." Conservatives have a majority there.

The negotiations began with Christian Democrat President Jose Napoleon Duarte's proposal at the United Nations Oct. 8 to meet with rebel leaders a week later.