Ethiopia warned both the United Nations and the Organization of African Unity today that if they failed to take action to balt the Somali "war of aggression" against Ethiopia they risk incurring the same fate as the now-defunet League of Nations before World War II.

Lt. Col. Mengistu Haile Mariam, the Ethiopian military strongman, charged at a press conference that Somalia had "arrogantly violated" the basic principles of both the African and world body by sending its regular armed forces into the Ogaden region of southeastern Ethiopia in an attempt to seize it by force and dismember the country.

"If the peoples of the world in general and those in Africa in particular do not pay attention to this fact," he said, "the history of the League of Nations may repeat itself."

The late deposed Emperor Haile Selassie made a stirring but unsuccessful appeal to the League in 1936 for help in halting the Italian invasion of Ethiopia, but the League failed to act and collapsed shortly afterward at the outbreak of the second world war.

Mengistu gave no indication whether he was planning to take the Ethiopian compliant against Somalia to the United Nations this fall. But he said the African organization and "others," apparently a reference to the big powers, had to see to it that Somali regular army troops withdraw from Ethiopian territory. Otherwise, he said, the war between the two neigboring Marxist states would continue.

Meanwhile, the government also called upon mechanics to volunteer their labor for the war effort. It had already mobilized all medical personnel, trucks and buses and all veterens of the armed forces under 60 years of age.

Mengistu, who is chairman of the ruling Provinsional Military Council said that "fierce fighting" is under way in the Ogaden and spoke of the "defensive Ethiopian effort" against the invading Somali forces around the three major towns in the disputed territory - Jigjiga, Dire Dawa and Harrar.

During the two-hour press conference held in the old National Assembly building, Mengistu handled a broad range of questions on the war in the Ogaden, the role of the Soviet Union in supplying arms simultaneoulsy to both Ethiopia and Somalia, the Cuban and Israeli presences here and the situation in Northern Eritrea Province where separatists are waging another war against the central governmnet.

[Somali President Mohammed Siad Barre said in an interview published in Kuwait today that Israeli troops are fighting with the Ethiopians in Ogaden. The newspaper Alrai Alaam quoted him as saying that Israeli experts are also training the Ethiopian army at Asmara, the capital of Eritrea province, Reuter reported from Kuwait.]

The Ethiopian strongman had bitter words for the Western press, which he charged with accepting the Somali contention that all the fighting was being done by the Western Somali Liberation Front rather than by regular Somali forces.

He said Ethiopia is willing to purchase arms from either the West or the East to protect its revolution and national unity but that the U.S. government had "closed the door on us." He did not elaborate.

It was understood, however, that he was referring to a considerable quantity of U.S. arms, including a squadron of F-5Es and a dozen M-60 heavy tanks, that were either on order or partly paid for when the Ethiopians summarily ended the American military aid program and unilaterally cancelled the 1953 American-Ethiopian defense treaty.

The U.S. position has been that this action put an end to the American-Ethiopian arms relationship for good and therefore there was no justification for going ahead with the delivery of the arms.

The slight, short, 37-year-old colonel spoke in a calm unemotional voice throughout the conference, which was his first major appearance before the international press since he became chairman last February.

Mengistu said that the Soviet Union had every right to provide arms to nations defending their territorial integrity but called up on it and all other nations providing war supplies to Somalia to reconsider in light of Somalia aggression against Ethiopia.

Mengistu said that reports of Cuban troops in Ethiopia were "malicious rumors" and that only Cuban "professionals," namely doctors and medical personnel, had come. He said Ethiopia intends to fight the wars against both Somalia and Eritrean separatists "without any forces from outside."

He skirted a question regarding reports of Israeli military assistance for Ethiopia, charging they were simply a pretext for the intervention by "reactionary Arab states" in the wars under way in the Ogaden and Eritea.