The United States had decided to send "lethal" equipment to the Salvadoran government as part of its renewed military aid to the embattled ruling junta here.

In a statement first released today in Washington, the State Department said that grenade launchers and M16 automatic rifles and ammunition would be included in a new $5 million package and that four Huey-type helicopters would be added to the two that arrived in the country this week on a special lease arrangement.

A justification for this escalation of U.S. military involvement here, the State Department cited both a depletion in government arms and ammunition stocks and the "covert" supply of arms by "Cuba and other communist countries" to Salvadoran guerrillas now waging an on-again, off-again offensive against the U.S.-backed government.

All U.S. military aid to El Salvador was suspended Dec. 5 amid allegations that Salvadoran troops were involved in the murder of four American churchwomen.

"Nonlethal" aid -- primarily transporation and communication equipment -- was renewed Tuesday and "lethal" aid begun today, the State Department said, because of "positive steps" in the investigation of the killings, the government's reform program and a reorganization of the military high command designed to replace officers most suspect of repressive abuses.

Strong criticism of the new aid package is expected from a number of Western governments opposed to U.S. support of the goverment, as well as human rights organizations that have charged that the Salvadoran armed forces are responsible for most of the thousands of deaths here in the last year of sporadic fighting.

Those governments, including many the United States consider allies among Western European and Latin American democracies, and the rights groups disapproved initially of the "nonlethal" aid renewal.

Although the State Department's Latin American bureau has strongly supported increased military assistance here, as have the Defense Department and the National Security Council, some who oppose it within the State Department have charged that its supporters are trying to curry favor with the more conservative incoming Reagan administration, and perhaps preserve their jobs in the transition.

The civil war here is at a virtual standstill. With this latest move from Washington, however, it is clear that a cold war is beginning to settle over Central America.

Publicly, U.S. officials say the line against communism in the area is being drawn, and privately they hint without providing evidence that the Soviet Union may be pressuring the leftist government of neighboring Nicaragua to supply more arms and become more involved in the Salvadoran conflict that its leaders would have wanted.

"Someone is playing throwaway with their little bulls --- country," is how one diplomat here described Nicaragua's situation.

Here in El Salvador, where the actual war is under way, the bloodshed that has now continued for more than a year has now become even more heavy in the last seven days.

The guerrillas began their "final offensve" against the government with a blaze of machine-gun bursts and the blast of rocket grenades across the country, the long-festering confrontation looks less final than ever.

With the war erupting in city slums and villages, around military garrisons and in the sugar-cane fields, the guerrillas have proven they can mount coordinated actions virtually anywhere in this overcrowded Central American country and operate almost freely in the rural areas. The government has shown it can hold the cities, but neither force can claim full control.

The international press that descended on this capital often finds itself unable to clarify or confirm much of what is happening. Despite relative calm in San Salvador, the countryside is so hostile and unpredictable that detailed accounts of the fighting are almost impossible to obtain.

Three journalists have been injured and one killed since the offensive began, and the Red Cross has estimated that 600 Salvadorans were killed in the week.

But first-hand observation and such reliable reports as exist indicate that the five guerrilla organizations in the Farabundo Marti National Liberation Front have been regrouping for the last two days and could resume heavy attacks at any time.

They are far from defeat or victory, having proved effective as soldiers but weak as popular leaders.

"The final offensive has begun, said guerrillas who seized two radio stations at the beginning of the fight last Saturday evening. They called on Salvadorans to help the insurgents, turn in government informers and fight with whatever they could.

But the popular response was at most apathetic. When the guerrilla organizations and their political allies in the Revolutionary Democratic Front called for a nationwide strike at the beginning of the week, few people joined them. After a fitful slowdown Monday and Tuesday, business was near normal by Wednesday.

"The offensive is gradually escalating," declared a Salvadoran Communist Party spokesman in Mexico. "The strike will grow as the military offensive advances."

The guerrillas' military tactics do appear to present a serious threat to the government, even without massive insurrection. They have worked steadily to cut the nation's two major highways.

Countless ditches were dug, scores of cars stopped and burned, to block the coast road and the Pan American Highway.

If the eastern sector, where the left is highly organized, and the area close to alleged supply lines from Nicaragua can be isolated from the more urban west, the long-promised guerilla government in Salvadoran territory might be established.

The left has shown itself remarkably well-armed, with U.S.-made grenade launchers and recoilless rifles, Chinese rocket-propelled grenades, Soviet fragmentation grenades and lethal homemade mines as well as a variety of sophisticated automatic rifles and machine guns.

But they have also lost a good deal of this equipment in battle and shown limited skill with the heavy weapons. Grenades launched at the military airport when the offensive began soared over their objectives and landed harmlessly in nearby fields.

The tactic of the goverment so far is to hold its position in the cities and towns. Some garrisons, without significant reinforcement, weathered sustained assaults for four days before the guerrillas fell back.

An initial split within the armed forces now seems controlled, although the left claims that a few of soldiers have deserted since the mutiny a week ago of some troops at Santa Ana.

Because the fighting on the ground here is a virtual stalemate, the propaganda and diplomatic offensives seem to take on more importance.

U.S. renewal of military aid to the Salvadoran government and charges that Nicaragua is supplying arms and at least a staging area to the Salvadoran insurgents could lead to a complete cutoff of U.S. aid to Nicaragua under current congressional restrictions.

U.S. diplomats here insist that they have firm evidence of Nicaraguan aid to the guerrillas, but none has been made public. A strong circumstantial case exists, however, after the landing of guerrillas -- perhaps 100 -- from the Gulf of Fonseca, which separates Nicargua from El Salvador. A confrontation followed but the government did not report taking any prisoners.

According to residents of the area, the government's claims that more than 50 guerrillas were killed are greatly inflated.

When the U.S. Embassy and the government released information on this action Wednesday, they implied that Nicaraguan troops might actually have been involved. Now both the government and the embassy are saying they believe the guerrillas who landed were probably Salvadorans who had trained in Nicaragua.

A team of five U.S. military advisors is working with El Salvador's Army and two U.S.-leased Huey helicopters already have arrived with six technicians: to get them flying within the next two or three days.

The fighting shows every sign of continuing for a long time, with periodic lulls such as that at the end of this week. In the meantime, industrialists -- who represent what would have to be the economic future of the country since with well over 500 people to the square mile, El Salvador has limited agricultural prospects -- are openly asking how long they can continue with the violence at its current level.

According to diplomatic sources, the Salvadoran goverment has made at least five overtures to the political wing of the leftist movement within the last three weeks, and been rebuffed each time. Guillermo Ungo, president of the revolutionary front, said several times that his side is willing to negotiate, but only with the United States, not with the Salvadoran government.