Six U.S.-loaned Bell UH1H helicopters, waiting to be transformed into gunships and troop transports, rest like slouching beasts near the runway of Ilopango Airport outside this capital. Salvadoran troops are breaking out their new American M16 rifles, grenade launchers, flak jackets and steel helmets.

Up to 20 U.S. advisers are showing them how to make it all work and keep working while another five are teaching their commanders strategy for hunting down battered leftist guerrillas retreating in the wake of their failed "final offensive."

The leftist push that began Jan. 10 took El Salvador's bloody political strife over the brink to civil war. With Guatemalan and Honduran troops on alert, Nicaragua accused of helping Cuba and other communist countries aid the guerrillas, and the conspicuous arrival of U.S. guns, the war is heading toward even wider and more dangerous confrontations.

State Department spokesmen in Washington carefully emphasize the "modest" nature of the new $5 million military aid program for El Salvador. But for the first time since El Salvador was caught in the net of Carter administration refusal to provide guns and bullets to alleged human rights abusers in Latin America in 1977, that aid now includes "lethal" military equipment.

Approved by Carter in the waning hours of his presidency, these shipments were justified to the public partially as military necessity and partly on the basis of Salvadoran government claims that it is trying to investigate the abuses, end them and punish their perpetrators. El Salvador also says it is planning to transfer government control from the military to civilian leadership.

But the resumption of military aid and its subsequent increase was based primarily on U.S. fears that the U.S.-backed government was about to be overrun by the leftists, according to analysts. Official U.S. claims of improvement here are, at best, misleading. Both public and private information indicates that Salvadoran investigations are either nonexistent or ineffective, that widespread abuses continue and that control remains largely in the hands of the military.

For the last two years, U.S. policy here has been to try to end the terrorism and fighting by undermining right-wing extremists, drawing the line against a violent takeover by the radical left, and attempting to foster a centrist government capable of carrying out sweeping reforms of the country's social, political and economic structures.

To shore up these efforts, but still discourage human rights abuses, the State Department has supplied El Salvador with massive economic aid -- at least $82 million last year -- and small amounts of military support limited to "nonlethal" transportation and communications equipment.

On Dec. 5, all U.S. military and economic support suddenly was suspended amid charges that Salvadoran government security forces were involved in the brutal murder of four American churchwomen, including three nuns.

Economic aid was renewed after 12 days, on the ground that the Salvadoran economy was about to collapse. But military support -- which church and human rights groups claimed would be an endorsement of Salvadoran military abuses -- remainded in limbo.

While the Salvadoran left is brutal, opponents of military aid argued, the United States does not identify itself with the left. It is already intimately identified with the government.

As late as Dec. 22, then secretary of state Edmund Muskie met with the families of three of the slain women and U.S. church leaders to assure them that military aid would not be resumed until an investigation of the deaths was carried out. Muskie promised the families they would be kept informed of progress in the matter.

By early January, however, there were reports of high-quality arms in large quantities reaching the guerrillas, and on Jan. 10 the left began its concerted drive to seize power. It was soon evident that the fight would be a direct military confrontation, and not a popular insurrection, as most Salvadorans ignored the guerrillas' calls for active support.

On Jan. 17, a Saturday night, the State Department announced its decision to provide more deadly support to the Salvadoran government.

According to the official U.S. statement, the action was taken because the Salvadoran government was taking "positive steps in the areas of mutual concern." It mentioned the investigations of the nuns' killing, the reforms to redistribute land and wealth, and efforts to curb repression and terrorism. It also said that "we must support the Salvadoran government in its struggle against left-wing terrorism supported covertly with arms, ammunition, training and political and military advice by Cuba and other communist nations."

According to U.S. Ambassador Robert White, however, there has been no progress toward discovering which, if any, Salvadoran military or other official personnel were involved in the killings.

"On the contrary," said one official close to the proceedings, "right now all the government is doing is finding excuses why they are not doing anything. aThe bodies of two of the nuns buried in Chalatenango still have not been exhumed. That is the kind of cooperation the investigation is getting."

The absence of any substantive investigation into the murder of the Americans is not unique in this country where more than 10,000 people were killed last year. In the rare cases where there are investigations at all, none, at least as far as the public is aware, has ever brought results. Despite strong leads in the case of the murder last spring of San Salvador archbishop Oscar Romero, the killer has never been found. One judge early on the in case fled the country after he hinted at evidence of right-wing military involvement. Nothing more has been heard of him or the investigation.

The reforms mentioned by the State Department are only half, and rather haphazardly, implemented.

The program of land distribution was designed to proceed in three stages. The first state -- government confiscation and redistribution to peasant farmers of large uncultivated estates -- has been carried out. But the peasants find they cannot get bank credits to buy the seeds and fertilizers that would keep production at least up to average. At the same time, the large landholders whose estates were seized last March have yet to receive promised compensation.

The second state, which would turn medium-sized farms into collectives, has not been started and according to government and diplomatic sources almost certainly will not be.

The third stage, which would give the land they work to sharecroppers and tenant farmers, has been paralyzed by government refusal to distribute titles to the land.

There is little evidence to suggest that the Salvadoran government has controlled the brutality of some of its troops. Prisoners are not taken and guerrillas supposedly killed in the heat of battle are found tortured, raped and burned. At a press conference last week the military commander, Jaime Abdul Gutierrez estimated that more than 1,000 guerrillas had been killed during the offensive. He said he believed about five had been taken prisoner.

If there are positive changes in the government, they may be seen in the restructuring last month that made Christian Democratic leader Jose Napoleon Duarte president. They did not, however, give him command of the armed forces.

Duarte moved during the offensive to reestablish the popular support he lost during seven years of exile and the last year of rabid political infighting.

Although additional changes have been made in the command structure of the armed forces and others are planned, they may not signify as much reform as some U.S. officials claim.

The most important change cited by Washington and the U.S. Embassy here was the transfer of Col. Nicolas Carranza from the post of vice minister of defense to a seemingly innocuous job as head of the government-run telecommunications system. But the communications job is like being "head of intelligence," one conservative Salvadoran said. "Which is something they [the military] didn't have before -- someone to take advantage of all this information."