NAIROBI, KENYA, NOV. 13 -- The international effort to head off famine in northern Ethiopia appears to be failing as large numbers of drought victims have begun migrating on foot in search of food, according to western relief officials in Ethiopia.

Rebel attacks on truck convoys, two of which occurred this week, have slowed to a trickle the delivery of food in the Tigray and Eritrea regions of Ethiopia where it is estimated that 2.1 million people are threatened with starvation.

"The rebels are stopping transport in the north at a critical time when all the food is almost out. It is really bad. The whole situation changed with the attacks," Rick Machmer, director in Ethiopia for the U.S. Agency for International Development (AID), said today.

Machmer visited relief food warehouses this week in Mekelle, capital of the drought-ravaged Tigray region. He said he found the warehouses "practically empty."

"The international Red Cross needs 5,000 tons of food in Tigray today. They have 110 tons. That gives you the dimensions of this problem. It is pathetic," Machmer said.

Early detection of drought in July, along with commitments in September of American and European food aid, had led relief officials in Ethiopia to predict that widespread starvation in the coming year could be averted.

Donors had hoped to distribute food to destitute farmers before they abandoned their land and migrated toward famine shelters. It was in overcrowded, disease-ridden shelters that hundreds of thousands of Ethiopians died in the 1984-85 famine.

For growing numbers of northern Ethiopians, however, this year's worldwide effort at famine prevention appears to have fallen short.

"We are already in it," one senior western diplomat said today. "It is too late."

Several relief agencies report that drought victims are now roaming the northern Ethiopian highlands in search of food and are beginning to gather near the sites of what in 1984-85 were huge famine shelters.

Ethiopia's chief famine relief official, Berhanu Jembere, told western relief officials last week that about 5,000 people had walked out of the highlands of the Wello region to gather near the village of Ibnet. Three years ago, more than 100,000 people lived there in a squalid camp. Jembere reportedly said the Ethiopian government has encouraged the destitute people to leave the Ibnet area.

Relief workers with the International Committee of the Red Cross (ICRC) and United Nations field officers report that 7,000 to 10,000 people have concentrated near the central highland town of Korem, the site of another large famine shelter in 1984-85. ICRC field offices also report that "tens of thousands" of people are on the move in highland areas looking for food.

The Ethiopian government today raised its assessment of the total number of people affected by drought to 5.2 million and said it will take at least 1.4 million tons of food to feed them in the coming year.

There is "plenty of food in the pipeline" to meet Ethiopia's needs for the next five months, according to Michael Priestly, head of the U.N. emergency operation in Ethiopia. The problem, he said today, is "the movement of that food to the hungry people in the north. The logistics of moving food were made very much worse by the diabolical attack on Oct. 23."

In that attack, responsibility for which was claimed by rebels of the Eritrean People's Liberation Front (EPLF), an unguarded U.N. convoy of 23 trucks carrying 450 tons of donated wheat was ambushed and burned. The convoy, its cargo intended for drought victims in Tigray, was traveling south from the Eritrean capital of Asmara when it was destroyed.

The rebels, locked in a 27-year-old civil war with the Ethiopian government, claimed that government soldiers were using the convoy as "camouflage" for moving weapons.

Priestly angrily denied the claim, saying the rebels "never even lifted the tarpaulins to see what they were burning."

This week's two guerrilla attacks, details of which are still sketchy, reportedly did not involve international food-relief trucks.

Relief officials in the Ethiopian capital, Addis Ababa, said the first attack, which occurred Wednesday, was against an Ethiopian Army convoy. The second, yesterday, destroyed a convoy of 10 private commercial trucks that reportedly were carrying food.

Rebel radio claimed yesterday that 85 government soldiers were killed in the attacks, which took place on the same road as the Oct. 23 attack against the U.N. convoy.

In Addis Ababa, U.S. AID director Machmer said the attacks this week may scuttle a planned American scheme to give financial incentives to private trucking companies that carry food in northern Ethiopia.

"No commercial trucks are going to take risks to move along that road now," Machmer said.

The guerrillas have demanded that donor agencies supply advance information on the movement of relief convoys inside Eritrea. U.N. officials in Ethiopia, who by international law are supposed to have no formal contact with rebel groups, have called the demand "out of the question."

Priestly vowed today that the United Nations will ignore the rebels' demand and soon will resume transport of food in unguarded truck convoys.

"We believe the situation is so serious in Tigray that we have to keep going with these convoys," Priestly said. "The guerrillas must see that any further attacks on these convoys will result in massive death. The rebels must take direct responsibility for the consequences."

A show of selectivity in targets in this week's guerrilla attack has given western relief officials some reason to hope that the rebels may have been stung by adverse international publicity over their attack in October. It also indicates that, in the future, the guerrillas may allow free passage of some relief vehicles.

According to Machmer, the rebels on Wednesday did not attack 14 Catholic Relief Service trucks carrying U.S.-donated food. The trucks were traveling on the same road and in the same vicinity as the Ethiopian military convoy when it came under rebel fire.

Rebel attacks on western relief convoys have backed the U.S. government into a corner regarding the future of humanitarian aid in northern Ethiopia. In addition to being the largest supplier of relief food to the Ethiopia through its leftist government, the United States is also the principal food donor for the rebels.

In what is supposed to be a secret operation, a senior western diplomat said the rebels are scheduled to receive 15,000 tons of U.S. food in the coming year. As during the 1984-85 famine, the food is to be delivered in rebel-controlled areas of Eritrea through the "back door," by way of neighboring Sudan.

The U.S. government has warned the rebels, however, that they will not get the food if attacks on relief convoys continue.

According to relief and diplomatic sources in Ethiopia and Sudan, the Eritrean rebels this fall have embarked on their largest offensive in the past decade. The attacks on the truck convoys, these sources say, are part of the offensive.