MASSAWA, Ethiopia (AP) - Day and night, heavy artillery and tank barrages thunder along a 90-mile front, in what might be the decisive battle of the nearly 30-year-old Eritrean civil war, Africa's longest-running conflict.

The battle raging between Ethiopian government troops and Eritrean rebels in Ethiopia's northernmost province is possibly the most destructive conflict in the world today, resulting in thousands of casualties on both sides.

The rebels appear to have the upper hand in the fighting along what is called the Ghinda front. So fierce are the artillery and tank exchanges that their distant rumble can be heard clearly in the strategic Red Sea port of Massawa, about 37 miles away.

Massawa, now in rebel hands, was the target of six Ethiopian air raids in April by Soviet-built MiG fighters. Rebel spokesmen say at least 110 people, many of them civilians, have been killed in the cluster bomb attacks.

The antagonists in the fight are the Marxist government of President Mengistu Haile Mariam and rebel forces dominated by the Eritrean People's Liberation Front, which seek independence for the province of 3.5 million people. The Eritrean Front is considered left-leaning, but its leaders say they reject any political identification tag.

The government forces' immediate objective is to recapture Massawa, one of only two Ethiopian Red Sea harbors that have traditionally handled most of the country's trade and international relief supplies for millions of famine victims.

The port was captured by the Eritreans on Feb. 11, three days after they launched a majo new offensive in their 29-year-old war of secession.

The rebels have their sights set on a bigger objective - the capture of their ancient, hallowed provincial capital of Asmara and the end to a conflict that has claimed hundreds of thousands of lives.

The Ghinda front straddles the only road from Asmara to Massawa, a two-lane ribbon of asphalt stretching 62 miles.

The Addis Ababa government has issued few war communiques, but in a rare pronouncement acknowledged last Friday that the battle had ''reached a decisive final stage.''

The last previous government statement on the conflict came in mid- February, shortly after the fall of Massawa to the rebels.

At that time, Mengistu told parliament that failure to recapture Massawa would mean the loss of his 2nd Revolutionary Army, Asmara, and Eritrea itself.

That now seems likely.

Mengistu's 2nd Army, more than 100,000 strong, representing almost a third of Ethiopia's military strength, is virtually surrounded in the highlands around Asmara. Its only remaining supply route is a tenuous air link from government-held territory far to the south and east.

Another major rebel force, the Tigre People's Liberation Front, has tied up Mengistu's remaining armies in an offensive that has taken the Tigreans to within 100 miles of the capital of Addis Ababa. The Tigreans are seeking Mengistu's ouster.

The Ethiopian government claims, and the Eritrean rebels freely acknowledge, that the two offensives by the separate rebel forces have been coordinated. The Tigrean rebels began their offensive last August, sweeping south and east out of their stronghold of Tigre Province, just south of Eritrea.

Isayas Afeworki, the general secretary of the Eritrean rebel movement, says his strategy is to bleed Mengistu's 2nd Army dry on the Ghinda front, then retake the offensive.

''There is no hope for the dergue (government) to take back Massawa,'' Afeworki said. ''I think what they are doing is motivated by desperation.''

A new offensive, aimed at Asmara from the south, already is under way, but it is unclear if it is being pursued by the Eritreans or the Tigreans.

Ethiopia claims a Tigrean force has pushed northward, overrunning three government garrisons south of Asmara and behind the Ghinda front. The Eritreans claim the new move as their own.

On the Ghinda front, Afeworki said the Ethiopian army had made three major attempts to retake Massawa in the past month, but described it as ''a futile, suicidal effort.''

Afeworki and other rebel leaders said the 2nd Army's only access to the lowland plains leading to the Red Sea is through a few, narrow mountain roads that are easy to choke.

''They have to get through our artillery, tank and mortar fire, then our minefields. Then our fighters pop up out of their bunkers and chew up those that remain,'' Afeworki said.

The rebels claim they have inflicted some 13,000 casualties on government forces in nearly a month of fighting along the Ghinda front, including 6,000 killed. They don't talk in specific numbers about their own losses, but Afeworki said they were less than a tenth of the government's.

Ethiopia's armed forces are the largest in black Africa, numbering 315,800. The International Institute for Strategic Studies, based in London, estimates the Eritrean rebel force at 30,000, and the Tigreans at 20,000.

The government and the rebels have made exaggerated battlefield claims which cannot be independently verified. But there is no doubt about the ferocity of the fighting.

In recent testimony to a Senate committee in Washington, the assistant secretary of state for African affairs, Herman J. Cohen, described the conflict as ''possibly the largest in terms of combatants and the most destructive in the world today.''

The intensified fighting follows two unsuccessful rounds of peace talks held late last year between government and rebel emissaries under the mediation of former President Jimmy Carter in Nairobi, Kenya, and Atlanta.

AP-DS-05-01-90 1349EST