After nearly six months of massive American aid to this famine-crippled country, relations between the U.S. and Ethiopian governments continue to be marked by recriminatory rhetoric and a tangle of red tape that restricts the movements of U.S. aid officials.

While there has been cooperation at the working level, relief officials said, even that could end because of expected Ethiopian opposition to a U.S. decision to step up "backdoor" shipments of food into rebel-controlled areas through Sudan.

According to senior international relief officials here, the United States continues to be singled out among donor nations for criticism by Ethiopia's Marxist military government. This week Ethiopia's leader, Mengistu Haile Mariam, spoke of the "belligerency of American imperialism" in the Horn of Africa.

In addition, one senior western aid official said U.S. relief employes here continue to be subjected to petty harassment from the Ethiopian government, such as week-long delays in obtaining travel permits, denial of long-term residency visas and frequent searches of their hotel rooms.

Although the U.S. government has become by far the largest donor to Ethiopia, pledging 416,000 tons of food worth $208 million and bankrolling one-third of all famine relief in the country, there has been almost no improvement in official relations, several senior international relief officials said here last week.

"The Americans thought that, coming in here with a massive program, at least the Ethiopian government would make it easier for them to operate," a senior western aid official said. "But it has not."

Thus far, strained U.S.-Ethiopian relations "have not impaired the flow of U.S. food aid to Ethiopians," according to Maurice Strong, undersecretary general for U.N. emergency operations in Africa. "Despite tremendous political and ideological differences, there is cooperation on the ground."

About one-fourth of the U.S. food pledged this year has arrived and is moving through private voluntary organizations and Ethiopian government channels. Relief officials here say the U.S. food has helped save tens of thousands of the estimated 7.7 million Ethiopians threatened by the famine.

U.S. and U.N. officials agree, however, that this cooperation will be tested severely by a decision of the Agency for International Development to move 100,000 tons of food through the "back door" by way of Sudan into rebel-controlled areas of northern Ethiopia.

Ethiopian officials repeatedly have warned that any movement of food into the northern provinces of Tigray and Eritrea through Sudan would be considered a violation of Ethiopia's sovereignty and an act of "aggression."

In its view, unauthorized food aid in the north is tantamount to support for the rebels there, some of whom have been fighting the Addis Ababa government for more than two decades.

Mengistu said last week that "under the cover of providing relief food aid to compatriots from the northern areas . . . imperialists were providing arms and supplies to the secessionists."

In the past year, according to western aid officials, the Ethiopian government officially has condemned -- but in practice tolerated -- the trickle of "backdoor" food aid to the rebel-controlled north. A massive new U.S. commitment to such aid, however, invites retaliation by the government, these officials say.

A likely casualty of the AID decision, officials here agree, could be U.S. efforts to get Ethiopian permission to move food aid already in the country to the rebel-controlled area.

The sensitivity here grows out of a historical preoccupation -- often mentioned by Mengistu and senior aides -- with threats by Arab neighbors to the traditionally Christian Ethiopian empire.

The northern and southern borders of Ethiopia, established by conquest around the the turn of the century, cut into traditionally Moslem cultures. Much of the history of Ethiopia since then has turned on the issue of Moslem rebellion and efforts by the Addis Ababa leadership -- historically Christian and not Marxist -- to keep the empire in one piece.

That "embattled empire" mentality, western diplomats here say, explains the Ethiopian government's antipathy toward the United States and its embrace of the Soviet Union.

Soviet arms and Cuban troops saved Ethiopia in 1977-78 by turning back an invasion in the south by Somalia. The Somalis, a Moslem people who are ancient enemies of the Ethiopians, had invaded just after the United States ended its longstanding military support of the Addis Ababa government.

"Mengistu feels that the Americans abandoned Ethiopia and the Russians saved his country," said a relief official who has met recently with Mengistu. He and other diplomats and famine relief specialists here said that the past six months of U.S. generosity have not changed Mengistu's mind.

Relief workers and officials who have traveled widely in Ethiopia in recent months say, however, that the antipathy of the government here toward the United States is not shared by the Ethiopian people -- neither the peasants in the countryside nor the middle class in the cities.