ADDIS ABABA, Ethiopia (AP) _ The first senior U.S. official to visit Ethiopia since its 1974 Marxist revolution arrived today for talks on this nation's 27-year civil war and stability in the Horn of Africa.

Herman J. Cohen, assistant secretary of state for African affairs, told reporters after arriving that he has ''a long agenda of issues'' he is prepared to discuss ''in a frank and fair manner.''

He is to spend two days in this strategic but impoverished nation that is the Soviet Union's staunchest ally on the continent.

That friendship with Moscow soured once-warm relations between Washington and Addis Ababa after President Mengistu Haile Mariam Mengistu and a clique of fellow military officers ousted the Emperor Haile Selassie in 1974 and the Soviet Union became Ethiopia's biggest patron in 1977.

Cohen was expected to meet Foreign Minister Berhanu Bayih this afternoon and Mengistu on Saturday before leaving Sunday for Sudan.

He said that besides Ethiopia's civil war and regional conflicts he plans to discuss economic issues and the return of refugees in the region.

He also was expected to discuss money the United States says Ethiopia still owes from weapons it purchased years ago. Discussions on economic growth will be handicapped by U.S. laws that deny development aid to countries that owe money for arms and are accused of human rights abuses.

Cohen's visit comes a day after Amnesty International published a report charging that Ethiopia is among the worst offenders in Africa in failing to ratify international human rights covenants.

The London-based organization's report said thousands of suspected political opponents in Ethiopia are imprisoned for months or even years without trial and that many are tortured. Others vanish without explanation, it said.

Cohen's concern for refugees involves hundreds of thousands who have fled wars in neighboring Sudan and Somalia and are living in refugee camps in Ethiopia, while large numbers of Ethiopians have fled to Sudan to escape their country's northern civil war.

Ethiopian refugees also remain in Somalia, where they fled a decade ago after Ethiopian troops backed by Soviets and Cubans repulsed a Somali invasion.

The American official's visit comes after several appeals from Mengistu for improved relations.

After he seized power, Mengistu's government threw out the U.S. Military Assistance Group and closed the American government's information service. Washington retaliated by suspending its technical assistance program, then the largest in Africa.

The United States and Ethiopia also withdrew their ambassadors, leaving charge d'affaires to head their embassies.

There was little contact until the 1984-85 famine, when the United States was the biggest donor of emergency food.

Cohen initially was to visit Ethiopia at the end of May but that trip was postponed by an attempted coup in Addis Ababa.

He arrived from Egypt where he met with President Hosni Mubarak and reportedly discussed the civil wars in Ethiopia and Sudan. Guerrillas in northern Eritrea want independence from Ethiopia while southern Sudanese want more autonomy. All the rebels belong to ethnic minorities.

The U.S. Embassy said Cohen is the first U.S. official to visit Ethiopia in the 15 years.