There is growing evidence, mostly circumstantial but still persuasive, that Cuba and Libya plan to help the leftist military government of Ethiopia in its struggle to put down secessionist rebels in its vital Red Sea province of Eritrea.

If they do, it will bring them into conflict with the Arab States of Egypt, Sudan, Syria and Saudia Arabia, all of which support the Eritrean liberation movement.

Cuban Primi Minister Fidel Castro just completed a 10 day visit of Libya as the guest of Libyan leader Muammar Qaddafi, who has been developing close ties with the Soviet Union.

[Castro and Qaddafi, in a joint comunique, denounced "imperialist maneuvers" against the Ethiopian rulers, according to Granma, the official Cuban Communist Party daily, Reuter reported from Havana.]

In bits and pieces, a picture is beginning to emerge of growing Cuban interest, still apparently limited, in northern and eastern Africa.

Immediately after leaving Libya, Castro went to two countries in the vicinity of Ethiopia. Yesterday he conferred with leaders of militantly leftist South Yemen. Today, in a puzzling move, he reportedly went to Somalia, which has been involved in long-standing territorial disputes with neighboring Ethiopia.

Little information about the purpose of Castro's visit to Libya has been made available and diplomatic analysts here and in Tripoli have had little to add to the official accounts. According to these accounts, Castro spent his time visiting factories and model farms, seeing the "new Libya."

He rode in parades, toured the country and attended a session of the Libyan General Popular Congress at the desert town of Sebha where, according to diplomats who watched the spectacle, he appeared bored.

During Castro's visit, the rest of the Arab and African world was preoccupied with the first Afro-Arab summit conference and apparently paid little attention to Castro. The anti-Qaddafi Egyptian media, normally encouraged to report at length on signs of Soviet or other Communist influence in Libya, ignored Castro.

Yesterday President Anwar Sadat of Egypt, an old foe of Qaddafi, gave an interview to the official Sudan news agency in which he strongly criticized "the increasing Soviet intervention, through Libya, to support the Ethiopian regime which is savagely repressing the Eritrean revolution." He said nothing about Cuba.

The current trip by Castro however, reflects Cuba's interest in the area. So does a statement by President Felix Malloum of Chad, which has lost thousands of square miles of mineral-rich territory to a Libya land grab. Malloum said he was not disturbed by the amity between Cuba and Libya because his own country has excellent relations with Havana.

It has been reported recently that there are some Cuban military advisers in Uganda, though President Idi Amin has denied it.

There are also, according to reliable intelligence reports, Cuban advisers working in South Yemen, only 50 miles across the mouth of the Red Sea from Ethiopia's naval base of Assab, on the Eritrean coast.

Monday, while Castro was in Libya, a leader of the Eritrean rebellion said publicly that the Addis Ababa government was planning to call for Cuban help to suppress the movement.

In an interview with the Associated Press in Beirut, Osman Saleh Sabbi of the Eritrean Liberation Front said his group has "solid information" that the Ethiopian regime has asked Cuba for help. He said an advance contingent of Cubans was already in Addis Ababa to help the government in its efforts to supprjess the secessionist rebellion that has been going on for years in the province and that has reportedly made substantial gains recently against demoralized Ethiopian forces.

Calling for American help to prevent Cuban intervention, the Eritrean raised the specter of Angola and suggested that the Arab oil states would be unhappy to see Cuban military forces at the Red Sea. Anxiety about the security of the Red Sea shipping lanes, vital to Saudi Arabia, Egypt and Sudan, has been in the forefront of Arab concerns recently.

The ostensible source of this concern has been reports of Israeli military activity off the Eritrean coast, but diplomatic analysts here believe the real target was Ethiopia itself.

At a summit conference in Khartoum last week, the presidents of Egypt, Syria and Sudan, now united in a joint political command that is hostile to Libya, stressed this point, calling the Red Sea on "Arab sea."

The rebels in Eritrea are considered Arabs, while the Ethiopians, whi annexed Eritrea in 1962, are not.

Ethiopia's delegate to the Afro-Arab summit, Ato Berlami, denounced this attitude on the part of the three Arab partners.

"There is a dream of making the Red Sean an Arab lake," he said. "The only part of the coast of that sea that is not Arab is in Eritrea, and the Arab countries want to put an end to that situation."

While he named no specific countries, he said that some Arab states that did not share this view of the Eritrean situation, apparently a reference to Libya. Ethiopia and Libya had already been linked by circumstance if not by design in their common hostility to Sudan.

The increasingly pro-Western Sudan is actively supporting the Eritrean rebels, and has been a bitter enemy of Libya since Sudanese President Jaafar Nimeri accused Qaddafi of engineering a coup attempt against him last July.

Officials of the Cuban embassy here were not able to comment on their prime minister's visit to Libya.