ADDIS ABABA, Ethiopia (AP) _ Relief officials said Saturday that rains, long awaited in parched Ethiopia, have spoiled an estimated 8,800 metric tons of grain stockpiled at the Red Sea port of Assab - enough to feed all of the nation's famine victims for two days.

Authorities acknowledge 4,000 to 5,000 of the 110,000 metric tons of wheat and other foodstuffs have rotted at the port. But the relief workers say there is evidence a current probe will show that 8 percent or more has been ruined.

One of the international relief officials, all of whom spoke on condition they were not identified, said the ruined grain would have fed the East African country's 10.8 million famine victims for two days.

A metric ton is equivalent to 1.1 short tons, the ton measurement used in the United States. Some of the grain is in unwalled sheds, but much of it is piled outside in huge mounds.

Ethiopia made an international appeal Friday for plastic sheeting and wooden palettes to protect the grain now clogging Assab.

Port congestion at Assab, Massawa and in neighboring Djibouti was expected to be aggravated by a further 200,000 tons of grain - arrival due within the week.

''The storage capacity of all three ports is strained while there is an imminent risk of food deteriorating after recent freak storms in Assab and Djibouti,'' Kurt Jansson, the U.N. assistant secretary-general supervising the relief effort, said in a statement issued Saturday.

But Jansson said there should be an improvement in the port situation because of recent directives by Ethiopian leader Mengistu Haile Mariam.

Last Thursday, Mengistu promised to mobilize 70 percent of the government's long-haul civilian trucks - an estimated 750 vehicles - and provide 150 army trucks to move out grain at a faster rate.

Representatives of donor governments said they now realize that more grain than Ethiopia can handle is on its way. Privately, they predict Jansson will ask them to halve shipments until July.

Jansson's statement said that even with sustained trucking, he did not expect major stock decreases before then.

If all 200,000 tons arrive, it would be enough to feed all of Ethiopia's famine victims for two months, although only about half of them are now being fed at emergency centers.

Since December, 500,000 tons in food aid has been offloaded and a further 580,000 tons has been pledged - a total of almost 1.1 million tons against estimated requirements in 1985 of 1.3-1.5 million tons, said Dennis Craig, a U.N. World Food Program spokesman.