Barred from U.S. military assistance because of its human rights record, Guatemala's hard-pressed Army is keeping itself supplied with vital equipment through loopholes in U.S. laws and the conversion of new American civilian helicopters to military use.

The Reagan administration is aware of Guatemala's end runs around congressional reservations and in some cases has run interference for it, as in the decision last spring to remove military trucks and jeeps from a list of equipment that requires the State Department to take human rights into consideration before approving a sale.

In addition to technical explanations for this change in the rules, Deputy Assistant Secretary of State Stephen W. Bosworth explained at the time that "The administration's objective is to make our security interests and our human rights concerns mutually reinforcing so that they can be pursued in tandem."

Since the 1977 aid cutoff to Guatemala, Congress and the State Department have sometimes heatedly debated official and unofficial Guatemalan requests for spare parts for nine Bell military "Huey" helicopters purchased before the cutoff. The parts still have not been authorized.

But in 1980 and 1981, the Guatemalans spent about $10.5 million on three Bell 212 and six Bell 412 civilian helicopters--the civilian equivalent of the military Huey--according to sources close to the deals. These were bought with Commerce Department approval that does not require Congress or the State Department to sign off or even be informed. At least two of these new helicopters are now mounted with 30-caliber machine guns.

At garrisons such as this one in northwestern Guatemala, amid the rough mountainous terrain where leftist and communist guerrillas have their strongest forces, all this new equipment appears to be essential to Guatemala's counterinsurgency campaign.

The Army moves its troops overland in U.S.-made 6x6 trucks and jeeps. Many are so new that their chalked shipping destination, "Guat," is still visible on their sides. A blue and white Bell 212 with door guns is used to strafe nearby guerrilla positions while a just-arrived 412, still with its "executive" interior, carries troops to the scene of a battle and evacuates the casualties. Meanwhile, the machine guns and ammunition are taken from a grounded Huey to be mounted later on another "civilian" machine.

Although Guatemala over the past several years has indicated to the State Department its desire to purchase specifically military-designated equipment, it has been advised in most instances by State that such requests would never be approved by Congress since the necessary human rights certification could not be made.

Late last year, two congressmen, Rep. Michael D. Barnes (D-Md.) and Stephen J. Solarz (D-N.Y.), advised the administration of their intention to restrict further sales of military equipment to Guatemala with specific legislative prohibitions. They agreed not to seek such legislation in exchange for a promise by Assistant Secretary for Inter-American Affairs Thomas O. Enders that congressional approval would be sought for any sales of equipment on the list of military items.

This week Barnes was asked in Washington whether he thought future sales of civilian helicopters to Guatemala with the full knowledge that they would be converted to military use was a problem. The congressman, who heads the House subcommittee on inter-American affairs, replied , "It would violate the spirit" of the agreement and would belie Guatemala's contention that the helicopters were intended for civilian use.

Barnes said he has the impression that the Reagan administration wants to renew a military relationship with Guatemala because of its fears the government may need help in its battle against guerrillas but is "concerned about the reaction on the Hill."

"That is a justified concern," he added.

Barnes raised the essential questions about arms supplies to Guatemala's government at hearings last summer after the trucks and jeeps were reclassified for sale to this country's Army.

"Have there been any improvements in the human rights situation in Guatemala?" Barnes asked State Department officials. "Are such improvements likely if security assistance is resumed? Do extraordinary circumstances exist which necessitate a resumption of security assistance notwithstanding Guatemala's human rights questions?"

The number of political murders here--many of them government opponents--jumped by more than 150 percent during the past year to what some estimates put as high at 500 a month. Although they angrily denounce human rights reports by international organizations, which charge that the murders of political opponents and union organizers are orchestrated in an annex of the presidential palace, some Guatemalan military officers readily concede that civilians, usually Indians and peasants, who stand between them and the guerrillas, often are considered unfortunately expendable.

State Department officials argue that the question of how likely it is that U.S. supplies will promote improvements in human rights, is unanswerable until a new security relationship is tried.

But the issue is most clouded by the changing nature of the Guatemalan guerrilla war, now in its third decade. As the fighting here intensifies, the question of human rights is increasingly obscured by the realities of all-out battle.

Guatemalan and U.S. officials insist that the guerrillas are receiving substantial arms shipments from, or at least with the help of, the Soviet Bloc, Cuba and Nicaragua. Certainly some of the guerrilla units are well-equipped, reportedly uniformed and carrying American M16s, Israeli Galil automatic rifles like those used by government forces, rocket-propelled grenades and the other munitions now commonplace on the Central American scene.

But the extent of these supplies is unclear, and at a recent Army exhibition of captured guerrilla arms, almost all were homemade. The more sophisticated stuff, officers said when questioned about this, had gone to government troops to be put to use on their side.

The Guatemalan government insists that it can survive by its own wits and with its own resources. But at the same time the high command believes the only way to fight the guerrillas is with massive troop concentrations. Senior officers say that the size of the Army must be increased to about 100,000 men from its present level of perhaps 22,000 or it will be unable "to control Guatemala." To do this would require either more money than Guatemala's battered economy can offer, or military aid.

If the fighting continues to escalate, end runs and civilian helicopters are not likely to be enough to keep Guatemala's troops supplied.

Israel has taken up some of the slack, supplying not only the Galil rifles now standard issue among combat troops, but cartridge belts, helmets and as many as nine highly mobile Arava transport planes mounted with gun pods.

The Israelis also recently opened a military communications school in Guatemala to train radio technicians.

Israeli Ambassador Moshe Dayan (no relation to the late Israeli leader) says that Israel will sell arms to anybody, even Nicaragua and Cuba, if they want to buy. "We need military sales," he said.

There have been reports of Israeli advisers here as well. Dayan answered these by saying, "Maybe there are Israeli persons here, but they are not with the Israeli Army and not with the embassy. We do not have even a military attache and we do not have advisers here."

Until recently Guatemala might have turned to Western Europe for help. Indeed it bought several helicopters from France over the past decade. But France's new Socialist government, and the European Community's concerns about Guatemala's insistence on pressing its territorial claims against newly independent Belize have virtually eliminated any chance of arms supplies from that direction, according to diplomats here.

Ultimately, if the Guatemalan Army is to carry out the plans it deems vital to defeating the leftist insurgents, at whatever cost to human rights, it must turn to the United States.

"At some point," said one Western diplomat, "the United States is going to have to make some hard decisions.