NAIROBI, Kenya (AP) _ A U.N. relief ship loaded with emergency food supplies has reached the northern Ethiopian port of Massawa, the first such vessel to arrive since rebels seized the facility a year ago.

The Norwegian-flagged ''Far Suez,'' with a six-member U.N. team and about 10,000 tons of wheat, arrived at Massawa on Tuesday, said Paul Mitchell, a spokesman for the Rome-based World Food Program.

Agreement to reopen the port was reached Dec. 18 after negotiations with the Ethiopian government and the rebel Eritrean People's Liberation Front, which captured Massawa in February.

The U.N. Food and Agriculture Organization estimates 4.5 million people are at risk of starvation in Ethiopia, most of them in the war-stricken northern Ethiopian provinces of Eritrea and Tigre.

Mitchell, speaking by telephone from Rome, said the U.N. ship made the two- day trip from Djibouti and will repeat the voyage about every 20 days. Djibouti is a small nation lying between Ethiopia and Somalia on the Red Sea.

The World Food Program has estimated that Ethiopia will need about 941,00 tons of emergency food aid in 1991. The FA0 estimate is even higher, at 1.2 million.

Massawa, on Eritrea's Red Sea coast, is considered critical to the relief operation because it provides easier access to those in need in Eritrea and Tigre than Ethiopia's other port, Assab.

The rebels and the Ethiopian government had reservations on reopening the port, fearing the other side might use it to further its war efforts.

The Eritrean rebels have been fighting a 30-year war for Eritrea's independence from Addis Ababa. They have captured most of the countryside except the provincial capital of Asmara and an enclave around the city.