The military government of Guatemala, hailed by the Reagan administration when it took power seven months ago as a hope for an end to political killings in the Central American country, has suffered two new blows to its fading prestige.

The London-based human-rights group Amnesty International yesterday made public a statement saying government forces have "massacred more than 2,600 Indians and peasants in a new counterinsurgency program launched after Gen. Efrain Rios Montt came to power in March."

The statement was issued as the leaders of two Guatemalan parties that had supported the coup that brought Rios Montt to power were in Washington to publicize their disillusionment with his government and their demand that the military begin the process of handing over power to an elected civilian government.

The issues of human rights and democratic rule are crucial to efforts by administration officials to obtain military aid for Guatemala. Military aid was suspended in 1977 following charges that the Guatemalan government was involved in widespread killing and torture of its opponents.

Since then Congress has watched Guatemala's human-rights record closely, and no further military aid has been appropriated.

The House Foreign Affairs Committee has approved $250,000 for military training, but the bill has not yet passed. The administration has also expressed a desire to sell Guatemala between $3.5 million and $4 million in spare parts for planes and helicopters. But sources on Capitol Hill said the proposed aid has failed to receive the backing of the House Foreign Affairs Committee as required under an informal agreement between the administration and the committee.

Meanwhile, the administration has decided not to block international bank loans to Guatemala on human-rights grounds as had been done in the past, State Department sources said. The officials said the loans would be considered on a "case-by-case" basis on their economic merits and would not be opposed "automatically." An official said, however, that the new policy has not yet been implemented in actual votes.

The Treasury Department, in a letter to the House subcommittee on international development institutions, said the administration is considering the new voting policy because of "indications of improvements in the human-rights situation in Guatemala."

The Reagan administration, which would like to see a strong Guatemalan military to counter what it describes as Cuban and Soviet intervention in Central America, in many cases has supported the Guatemalan government's contention that massacres of peasants in the northern part of the country have been committed by leftist guerrillas.

Amnesty International and other human-rights groups say that the Guatemalan armed forces are responsible for most of the killings of unarmed civilians in the countryside.

In an unusual high-level response to Amnesty International's charges, Thomas O. Enders, assistant secretary of state for inter-American affairs, sent a four-page letter last month contesting a report on Guatemala by the human-rights group.

The Amnesty report accused the Guatemalan armed forces of "widespread killings, including the extrajudicial execution of large numbers of rural noncombatants, including entire families, as well as persons suspected of sympathy with violent or nonviolent opposition groups."

Amnesty International's report listed 112 incidents between March and July in which men, women and children were murdered in rural areas. The number reported killed in each incident ranged from one or two to as many as 100.

Enders said in his letter that the U.S. Embassy in Guatemala had been unable to corroborate many of the incidents listed by Amnesty and had differing information about other incidents. In seven specific incidents listed by Enders, he said the embassy findings supported the government contention that guerrillas were responsible for the killings.

The letter said two State Department officials visited the village of Salacuin in Alta Verapaz Province about three months after a reported massacre, and "confirmed the embassy account" that "guerrillas killed 26 people, including a mother nursing her infant."

Enders also said he assumed that many of the incidents listed by Amnesty International had been reported by groups he described as "closely aligned with, if not largely under the influence of, the guerrilla groups attempting to overthrow the Guatemalan government."

Larry Cox, a spokesman for Amnesty International, said the group's researchers obtained first-hand information from a "full range" of sources, including missionaries, medical personnel and anthropologists living in Guatemala, defectors from the military and "numerous" survivors of the reported attacks.

Amnesty International's office in London, in a statement referring to Enders' charge that the human- rights group took information from organizations linked to the guerrillas, said: "Amnesty International does not regard the political views of any source, or the fact that a source may be considered suspect by another party to the conflict, as reason for refusing to consider testimony."

The last official Amnesty International mission to Guatemala was in September 1979. Cox said yesterday's update on the massacres was released in connection with the organization's annual "Prisoner of Conscience Week."

Amnesty's July report emphasized that all peasant eyewitnesses who have reached "comparative safety abroad" have blamed the armed forces, rather than the guerrillas, for the massacres they saw.

Meanwhile, Vinicio Cerezo, secretary general of the Guatemalan Christian Democratic Party, and Renan Quinonez, secretary general of the National Renovating Party, met last week with Craig Johnstone, director of the State Department's Office of Central American Affairs, to discuss the demand by four major political parties that the military hand over power to a "democratic and pluralist" civilian government.

The two moderate parties have made common cause with two right-wing parties in demanding a "return to constitutionality" and refusing to participate in the newly formed Council of State, a civilian advisory board. Leaders of the four parties have signed a document urging political freedom, "free and clean elections" and the "eradication of violence and all types of paramilitary armed groups."

All four parties had supported Rios Montt when he first came to power. The two right-wing parties that signed the "Democratic Commitment" statement are the National Liberation Movement and the National Authentic Central.

The government responded to the parties' refusal to participate by decreeing censorship over all reporting of political activity in addition to the censorship already in effect restricting reporting on the war against the guerrillas to official military communiques.

Cerezo said in an interview that he had met with the Inter-American Human Rights Commission during its visit to Guatemala last month and given testimony that kidnapings and killings of political figures appeared to have resumed recently after stopping during the first months of the new government.

Guatemalan Ambassador Jose Zelaya Coronado denied the latest charges by Amnesty International, saying soldiers have "very precise instructions from the chief of staff to respect civilians."