Cuban President Fidel Castro left have today as secretively as he arrived two days ago, with no indication of what he had discussed with Ethiopia's radical military leaders other than "bilateral issues" and "major international issues of interest to the two countries."

Diplomatic sources here reported that Castro heard a pressing appeal from the chairman of the ruling Ethiopian Military Council, Lt. Col. Mengistu Haile Mariam, for Cuban military support to help defend his regime and the country's fledgling socialist revolution from its internal and external enemies now mounting a major offensive against them.

If so, there was no indication of what Castro may have decided. There was no joint communique published at the end of his visit and Castro made no comment in public that might signal his intention of helping the Ethiopian military government other than lending it moral support.

Castro's whereabouts after he left Addis Ababa were equally as secretive. He was scheduled to go to Dar es Salaam, Tanzania, but the official Tanzanian radio said he had postponed his arrival until Thursday. There was speculation here that Castro may be stopping in Uganda to see President Idi Amin. There have been unconfirmed reports of Cuban advisers and technicians in Uganda.

But the Cuban leader reportedly went no further in his toast to signal any concrete support for Col. Mengistu or the Ethiopian military government.

There have been reports circulating in African and Western diplomatic circles here that Ethiopia has asked for Cuban troops to be sent here, possibly from Angola or South Yemen, to bolster the military council under Col. Mengistu in its current "life and death struggle" with leftist and rightist opposition groups.

Most of this speculation regarding possible Cuban intervention stems from a visit early last month by the main Cuban military commander in Angola, Gen. Arnaldo Orcha. Among the places he visited was Asmara, capital of Eritrea Province in northern Ethiopia where a fierce secessionist struggle is under way and the Ethiopian army is being increasingly hardpressed to hold its own.

Some diplomats believe that Casto may have agreed to send several hundred Cuban advisers to help train Ethiopia's "people's militia" and also to bolster its internal security forces, but so far there is no hard evidence that more than a few such advisers have arrived.

The main reason for Castro's unannounced visit may have been an attempt to mediate between Thiopia and neighboring Somalia, two Marxist states that are-deeply at odds over a territorial dispute and over which country will gain a predominant influence in the French Territory of the Afars and Issas. The strategically located territory, sandwiched between the two countries at the mouth of the Red Sea, is a French colony that is scheduled to become independent this summer.

Somalia regards both Ethiopia's eastern Ogaden region and the French territory as parts of "greater Somalia" because of the presence of large numbers of Somail-speaking peoples in both area. Ethiopia rejects any changes in Africa's so-called colonial boundaries and vehemently opposes Somalia's claims to the French territory because its main outlet to the sea is the port of Djibouti in the tiny French colony.

(Col Mengistu, in an interview published Tuesday by the official Cuban daily Granma, advocated an alliance between Somalia, South Yemen and Ethiopia, together with an independent Territory of Afars and Issas, as a "common anti-imperialist front," Reuter reported from Havana.In the interview, Mengistu also accused the United States of encouraging Sudan to give shelter to deposed Ethiopian aristocrats and of providing weapons to the liberation movement in Eritrea).

The Soviet Union, presumably joined by Cuba, has been urging some kind of federation of the two socialist countries as a possible way out of the current impasse. There has been little indication so far, however, that either side accepts such a solution or that Somalia is ready to give up its claim to the Ogaden region.

In fact, Somalia backed insurgents are more active than ever in southern and eastern Ethiopia and there has been a steady deterioration in the socurity situation throughout the Ogaden over the past six months.

Castro's visit to Addis Ababa took place amid extremely tight security and at not time did he appear in public in contrast to the large mass rally held in his honor in Mogadiahti, the Somali capital, earlier this week.

During the two nights he spent in the Ethiopian capital, there was widespread shooting between pro-and antigovernment forces and several score of people were unfofficially reported killed. Some of the clashes took place near the national palace, the former residence of the late deposed Emperor Halle Selassie where Castro was staying.

Meanwhile, the Ethiopian media reported another 256 "counterrevolutionary outlaws" killed in fighting mostly in Harrarghe and Sidoma Provinces in southeastern Ethlopia, where Somali-supported insurgents are now operating in large numbers.

There is intense fighting under way between government forces - the army and the people's militia - and various opposition elements in at least seven of Thiopla's 14 provinces.