KHARTOUM, SUDAN, FEB. 12 -- Relief officials expressed concern today that Ethiopia's apparent renewed drive to deport peasants from rebel-held Tigray province may trigger the flight of up to 500,000 refugees to neighboring Sudan unless Addis Ababa abandons its forced resettlement plan.

Such a mass exodus, involving 2 1/2 times more Tigrayans than fled here from famine and forced resettlement in 1984-85, has become a distinct possibility following the reported killing of at least 20 peasants Monday by Ethiopian troops. Foreign relief sources said the civilians had refused orders in the Tigrayan city of Korem to board government trucks headed for resettlement farther south.

{The Ethiopian government denied the report Friday as an "outright lie," The Associated Press reported.}

Endrias Gebre, spokesman here for the Relief Society of Tigray, appealed to western governments, international humanitarian organizations and private relief groups to persuade the Ethiopian government to abandon forced resettlement.

No western relief organization here formally echoed that call. But representatives of humanitarian groups suggested privately that Tigrayans may start their four- to six-week trek on foot to Sudan without waiting for the outcome of diplomatic intervention or formal marching orders from the Tigray Peoples Liberation Front (TPLF), which has been fighting the Marxist Ethiopian government since 1974.

Last week, two prominent rebel leaders said here that if Ethiopia resumed forced resettlement, the TPLF would first issue a warning and then effectively set the exodus in motion by stopping all food shipments along government-held roads, according to relief workers.

The relief agencies' reluctance to speak out reflects a radical change in official Sudanese attitudes. Proud of its liberal asylum policy under President Jaafar Nimeri, who was deposed in 1985, Sudan and its troubled democratic regime now are unwilling to accept more refugees.

The government of Prime Minister Sadiq Mahdi has refused to allow its own refugee commission or foreign relief workers to make formal contingency plans to handle more than 100,000 refugees from Ethiopia, according to informed sources, although unofficial preparations are going ahead.

The principal bottleneck, relief workers said, has been government reluctance to authorize installation of water supplies in various locations where refugees were housed during the 18 months when 200,000 Tigrayans and an equal number of Eritreans were given asylum in 1984-85.

Eventually, 165,000 Tigrayans returned to their homes, according to Tigrayan relief agency statistics.

Relief officials suggested that the Sudanese government would like Tigrayan refugees kept out of sight at the inacessible border camp at Wad Kowli rather than brought farther inland where their presence could become a political problem.

But Wad Kowli typically runs short of water in the months before the rains begin in April, and food must be stockpiled because the site on the Gash River is often marooned after the rains start.

Tigrayan relief officials estimated that 13,000 Tigrayans died in Sudan in 1984-85, largely because of overcrowding, unsanitary conditions and the weakened state of those who had walked hundreds of miles from their parched mountain villages to avoid famine and resettlement.

Bad memories of Wad Kowli, and other camps where refugees were housed then, prompted the Tigrayan relief agency and American and European humanitarian organizations to truck food recently from Sudan into Tigray to prevent any massive new exodus and its dislocating social effects.

Before the shooting incident in Korem, the Relief Society of Tigray and the TPLF sought to expand these cross-border feeding operations to stockpile grain inside Tigray before the rains began. They hoped to acquire 130 trucks to add to their 170-truck fleet, but so far they have received only 40.

About 165,000 Tigrayans already have moved from the parched eastern and central regions to around the Tebeze River in the west where the relief society purchased grain locally.

The rebels still hoped to persuade Ethiopian officials to permit the Geneva-based International Committee of the Red Cross to distribute food without government military interference in a greatly expanded number of distribution points rather than the three on the main highway from Asmara, the Eritrean capital, to Addis Ababa.

Even before the renewed resettlement drive, Ethiopian officials sought to use food to control the population by limiting food distribution to three main centers under government control.

In the struggle to maintain their hold on the population, the rebels acknowledged that even stepped-up cross-border food shipments could not provide enough grain to feed the 1.1 million Tigrayans that the relief society estimates require famine rations.

At best, cross-border supplies and locally purchased food could feed half that number, according to relief workers.

The rebels' reluctance to approve an exodus reflects fears that refugees, once in Sudan, would miss the late spring and summer planting season, thus ensuring their absence for two years.