Robert White, former ambassador to El Salvador, spoke out strongly yesterday against military aid to the Salvadoran government, saying that weakened leftist guerrillas are no threat and that new military equipment would inevitably be used to "assassinate and kill in a totally uncontrolled way."

In dramatic testimony before the House Appropriations subcommittee on foreign operations, White, a career Foreign Service officer summarily dismissed by the Reagan administration last month, said "the chief killer of Salvadorans is the government security forces."

He cited the recent murder of four American church women and the execution of about 5,000 leftists and persons "merely suspected of being leftists."

At one point, White asked Republican congressmen voicing support for military aid, "do you want to associate yourself with this kind of killing?"

White's testimony was the most forceful public rebuttal thus far to administration insistence that leftist guerrillas backed by the Soviet Union and Cuba represent a threat to the survival of the Salvadoran junta. The remarks were his first public statement since his dismissal, which he called an act of vengenance" by the administration.

White's assertions are certain to add to the controversy over whether the United States should provide $25 million to $30 million in military aid and send as many as 26 military advisers to El Salvador. That option is now under consideration at the White House.

John Bushnell, acting assistant secretary of state for Latin American affairs, responded to White's testimony, saying "it baffles the imagination," given the amount of communist-supplied arms, that there would be no need to strengthen government forces.

Bushnell stopped short of requesting more military aid, but he noted that the $10.4 million provided by the Carter administration has been spent. "Although the miliary in El Salvador did very well" against the guerrillas, he said, "infiltration continues, and there will be some political needs that will arise."

Whie voiced firm support for the present coalition government, despite what he called a "miserable record" of repression. He praised government land reform which, he said, has compensated previous owners, set up agricultural cooperatives and is moving toward creation of family farms for almost 150,000 new owners in a country whose small oligarchy has maintained a stranglehold on the economy for generations.

The present government is "perfectly capable of handling the situation" without U.S. military aid, White said. "The leftist guerrilla threat has always been misanalyzed by the Pentagon . . . when the guerrillas announced their final offensive in January and imported massive quantities of arms, this beleaguered government put down the offensive without one cartridge coming from the United States . . . There is no reported incident of guerrillas holding any base or town for more than 24 hours."

When the guerrillas called for a general strike, everyone ignored it, he said.

Though acknowledging the flow of arms through Nicaragua into El Salvador, White said, "There are arms, but there aren't the guerrillas willing to pick them up because the cause of the guerrillas is declining and the cause of democracy is going forward."

White said that during his 10 months as ambassador, the Pentagon continually tried to increase the number of military advisers in El Salvador, which has been steady at five to six persons.

On Jan. 19, White said, Col. Eldon Cummings, commander of the U.S. military group in El Salvador, tried "under severe pressure from the Pentagon" to persuade him to send a message to Washington requesting 75 military adviser. White refused.

White also said he had opposed President Carter's resumption of miliary aid after the four American church women were murdered. He said the State Department claim that the government was making progress investigating the murders was false. "There never was, and there never has been a serious investigation of the deaths," he said.

White told reporters after the hearing that on at least two occasions he had recommended that the government use all of its resources, including the FBI and the IRS, to investigate the activities of Salvadorans in Miami.

They "provide important amounts of money to the security forces and right-wing hit squads," White said. "When you talk about assistance from the Soviet Union, you have to worry about the destabilizing effect of this flow of funds from Miami and Guatemala City."

White added that Salvadoran buisnessmen have been told by Miami exiles that unless the businessmen close their factories and leave El Salvador, their buildings would be torched. "The object of the right wing is to break the economy of El Salvador," White said.

Military aid, White said, would undercut "a fledgling government headed by civilians who are desperately trying to bring a recalcitrant military under control."

An injection of U.S. arms would increase random killings by government forces, White argued, resulting in a surge of poplar support for leftists. That would provide an excuse for a rigt-wing coup, he predicted.

Democratic congressmen at the hearing drew parallels between El Salvador and Vietnam, where hostilities involving the United States started with introduction of military advisers.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.), emerging from a meeting with President Reagan, said it would be "entirely appropriate for this country to dispatch noncombat advisers in small numbers -- 50, 100, 150 -- to tell these people how to defend themselves against Cuba."

Baker dismissed the comparison to Vietnam: "I do not favor the commitment of American troops in Central America. I do not believe President Reagan does."