Between 5 million and 6 million Ethiopians will need nearly a million metric tons of food aid as the country's famine grinds on into 1986, the U.N. Office for Emergency Operations in Ethiopia announced today

The U.N. assessment of food need, based on a just-completed crop assessment by the Food and Agriculture Organization, means Ethiopia needs almost exactly as much food next year as it received from this year's billion-dollar international aid operation.

By the end of December, about 1 million metric tons of food will have arrived in Ethiopia's ports this year.

Michael Priestly, newly appointed U.N. assistant secretary general for emergency aid in Ethiopia, said here that the 1986 need estimate will be dispatched immediately to the 35 governments and more than 40 private agencies that assisted the country in the past year.

The U.S. government will supply one-third of the 950,000 metric tons of food that the United Nations today described as Ethiopia's food need in 1986, according to Fred C. Fischer, head of the U.S. Agency for International Development operation in Ethiopia.

Other donors, Priestly said, will need to study the food requirement before pledging aid. The European Community this year provided about one-third of Ethiopia's food aid.

The U.N. food-aid assessment is "quite compatible," Priestly said, with an estimate announced in October by the Ethiopian government. That assessment said 5.7 million people will be threatened by famine next year and 1.2 million metric tons of food will be needed to feed them. Priestly said the differences between the two estimates were not significant.

For this year the government estimated that 7.9 million people were affected by famine and that they needed 1.5 million metric tons of food. Although only about 1 million metric tons of that request was delivered, the United Nations said about nine of 10 famine victims are now receiving regular rations of food.

Ethiopia's 1985 harvest will be about 20 to 25 percent better than last year's disastrous crop, according to the FAO crop assessment. But the FAO said it will still be about 13 percent below normal.

The reasons for what one FAO agriculture specialist called this year's "middling" crop, which is now being harvested, are sporadic rainfall that ended too early in many areas, late planting and shortages of seed and oxen. Many farmers were too weak from hunger to plant seed and care for their crops, the specialist said.

Only one area of Ethiopia, the Harerge region in the east of the country, will be significantly worse off in 1986 than it was this year, according to the United Nations.

As it was this year, Ethiopia in 1986 is likely to be the hardest hit of Africa's drought-prone countries. The FAO has said that good harvests in Sahelian countries such as Mali, Mauritania and Niger have all but eliminated their need for food aid in the coming year.

In another development in Ethiopia's continuing transportation problem at its ports, relief officials here said a barge carrying about 350 metric tons of U.S.-donated food, worth about $56,000, recently sank in the Red Sea port of Assab where it had been waiting nearly three months to be unloaded.

The International Committee for the Red Cross, which was to distribute the food, said it is in danger of losing thousands more tons of food unless the barges are unloaded more quickly.

The U.N. emergency office here said, however, that a growing fleet of trucks in Ethiopia has almost doubled the amount of food hauled out of Assab in the past month.