Eritrean rebel forces are bracing for a broad summer offensive by the Ethiopian Army despite recent separate attempts fostered by Sudan and the Soviet Union to settle their long secessionist war through negotiations on autonomy.

The expected Ethiopian drive appears designed to crush once and for all the already crippled guerrilla movement, which has been fighting for 18 years for the independence of Eritrea Province, stretching the length of Ethiopia's strategic Red Sea coastline.

Decisive victory would rid the Marxist government of Mengistu Haile Mariam of one of the most persistent problems it has faced since seizing power in 1974. A blow to the secessionists' military power also would enhance Ethiopia's value as a dependable Soviet ally along with South Yemen at the Bab al Mandab entrance to the Red Sea and a short flight from the oil fields of Saudi Arabia.

Ethiopia's strategic location is seen as the main reason for the Soviet-backed reconciliation attempts, which include meetings between rebel leaders and Mengistu's envoys in Berlin, Rome and Moscow. At the same time, President Jaafar Nimeri of Sudan has begun a mediation attempt backed by Egyptian President Anwar Sadat and President Mohammed Siad Barre of Somalia, both aligned with the United States in the struggle for influence in northeast Africa.

Eritrean rebel officials here believe Nimeri's motives center on the flood of refugees arriving in Sudan from Eritrea in the north and Uganda in the south. The Eritreans alone are said to number nearly half a million, severely straining Sudan's ability to provide food and shelter.

Ethiopian authorities have tucked about 2,500 Ethiopian families into Masawa, the main port, during the last two months to take up residence in homes vacated by some of the Eritreans who became refugees, rebel officials said.

Nimeri, whose vice president recently visited Addis Ababa, has announced that his borders are closed to rebels for operations against Ethopia. This reversal of a long history of aiding rebel logistics was interpreted as the price he had to pay for better relations with Ethopia, also in hopes of easing the refugee burden.

But Taha Nur, foreign relations chief of the Eritrean Liberation Front-People's Liberation Forces, said the Sudanese-Ethiopian accord in fact covered only those parts of the border that do not touch Eritrea and that rebels continue to operate from Sudan, "without any change at all."

The three main rebel factions, each using a variation of the name Eritrean Liberation Front, are bitterly divided on the suggestion that they negotiate some form of autonomy with Addis Ababa and abandon their long guerrilla war. But their leaders agree that, whatever the wisdom of negotiations, Mengistu's regime is preparing large-scale attacks in the rebel-held Eritrean countryside even as it shows willingness to discuss self-rule.

Ethiopia's Soviet-supplied military in recent weeks has increased sharply the amount of military equipment shipped into Asmara, the provincial capital, and called several meetings of high officers there to plan for the offensive, an Eritrean official here said today.

"The airport at Asmara is completely tied up with military transports," said Nur. "They are delivering arms daily."

Officials of the other principal groups -- the Eritrean Liberation Front-People's Liberation Forces, the Eritrean People's Liberation Front have made similar predictions in the last few days in Beirut and Rome. A spokesman for the Revolutionary Council said fresh fighting broke out last week northwest of Asmara in the village of Addi Kolom and Hadish Addi on the road linking Asmara with Keren and the Sudanese border.

Such attacks on Ethiopian military convoys have been the main form of guerrilla harassment since 16 months ago when Ethiopian forces backed by Soviet and Cuban advisers drove the rebels from Keren, their last major stronghold. That left the secessionists in control of only one Eritrean city, Maqfa, about 100 miles north of Asmara in the northern highlands.

"But the countryside still belongs to the revolution," insisted Nur. "And every day there are clashes."

The loss of Keren was widely considered as the collapse of the rebels' drive to win control of Eritrea. As predicted then, they have reverted to the rural guerrilla warfare with which they began their uprising after the late emperor Haile Selassie annexed Eritrea in 1962. Even in the countryside, their control is centered on the mountainous and inaccessible terrain around Naqfa, with occasional forays south to attack government targets.

Nur's office here issued a statement calling on Ahmed Nasser's Eritrean Liberation Front-Revolutionary Council, to halt its Soviet-sponsored contacts with the Addis Ababa government and declaring that only in negotiations including the United States, the Soviet Union and all three rebel groups can Eritrea's future be settled.

"Any Eritrean organization that meets representatives of the Ethiopian military government and negotiates with them the future of Eritrea will be considered from the national viewpoint as in violation of the political programs declared by the three Eritrean fronts," the statement said. "It also will be considered equal to betrayal of the national strategical aim of the Eritrean people's struggle to gain national independence."

His criticism reflected ethnic, ideological and personal differences dividing the three rebel groups.

Nur's Eritrean Liberation Front-People's Liberation Forces is headed by Osman Saleh Sabbe. It is considered favorable to Western countries, Egypt and Saudi Arabia. His forces receive funds from the anticommunist Saudi royal family and light arms, medicines and doctors from Egypt, rebel officials said.

The third group, the Eritrean People's Liberation Front of Mohammed Nureddin Amin, is said to oppose sponsorship from Soviet- or Western-oriented nations with equal vigor, reluctant to get the Eritrean movement involved in Arab disputes. In addition it is said to have considerable support from Eritrea's Christian population.