ABIDJAN, IVORY COAST, JUNE 3 -- The agreement between the United States and Soviet Union to launch an unprecedented joint food-supply operation to northern Ethiopia signals a major breakthrough in international efforts to bring relief to a famine-plagued, war-ravaged region in which an estimated 5 million people are threatened with starvation.

The agreement, reached during the just-concluded meeting between President Bush and Soviet leader Mikhail Gorbachev, underscores growing concern in the devastated region as the onset of Ethiopian rainy season threatens to make many roads to the hardest-hit areas impassable.

In a joint statement, the two powers -- whose Cold War policies were played out in disastrous regional conflict in the Horn of Africa over the last two decades -- agreed Saturday to "work together and combine their assets" by transporting an unspecified amount of U.S. food to Ethiopia aboard Soviet aircraft.

The U.S. and Soviets also agreed to seek a special United Nations conference of governments to try to mediate an end to the East African nation's 30-year-old civil war.

The U.S.-Soviet agreement came in conjunction with an announcement by the Marxist Ethiopian government of President Mengistu Haile Mariam that it is willing to allow food relief to pass through the Red Sea port of Massawa. The port, considered crucial to effective U.N. relief operations to the drought-stricken provinces of Eritrea and Tigray, has been out of operation since February, when the latest round of fighting began between government troops and rebels of the Eritrean Peoples' Liberation Front. The rebels have held the port for three months, but it has suffered extensive damage under government bombing.

The Soviet Union, for more than a decade the chief economic and military benefactor of Mengistu's government, has sharply cut its support in recent years. Faced with shortages of equipment and beset by low morale, the beleaguered regime has suffered sharp setbacks in battles with Eritrean and Tigrayan rebels since last autumn.

The Eritrean rebels are fighting for the region's independence, while the Tigrayans are fighting to overthrow Mengistu. The government charges that the two rebel groups have fought in tandem recently.

The Ethiopian fighting has continued with little respite in the last few weeks, and rebels claimed today that they had killed 1,200 government troops in two recent battles.