Largely unnoticed by the outside world, Ethiopia and Somalia have been engaged once again in large-scale fighting for the last six months for control of the Ogaden, over which the two neighboring nations have been warring almost continuously for 20 years.

Once again, too, the Ethiopians are prevailing, but this time without the help of Cuban combat troops, who played such a prominent role in the 1977-78 war between the two and raised such alarm in Western circles about Soviet expansionism in Africa.

After several years of being bottled up in garrisons and hardly moving beyond into the surrounding countryside, the Ethiopian Army, reinforced by two new divisions, has gone on a major offensive against Somali regular forces and guerrillas of the Somali-backed Western Somali Liberation Front.

Not only has it finally succeeded in clearing most of the northern and central portions of the Ogaden of hostile forces, it has for the first time in five years succeeded in reaching the disputed, colonial-drawn border line.

As the fighting has escalated and the Ethiopian presence made itself felt once more in the barren Ogaden landscape, thousands of Somali nomads and even some weary liberation front guerrillas are streaming into the already overcrowded shelters set up by the Ethiopian government for drought victims earlier this year.

The one here on the outskirts of this dusty crossroad market town now holds 15,000 Somalis and 300 new arrivals during a three-day period could be seen recently milling around the feeding center set up initially by the Ethiopian Relief and Rehabilitation Commission for drought victims.

The account of which country is responsible for the escalated fighting differs sharply on this side of the border from that on the Somali side. The Ethiopians say the Somalis started it last fall by stepping up the infiltration of regular and irregular forces to challenge a new Ethiopian control over the disputed territory while the bulk of their forces as well as their energies were concentrated in fighting separatist forces in northern Eritrea and Tigre provinces.

Despite Somali assurances to the U.S. government last August that their regular Army troops would no longer be involved in the Ogaden struggle, both Ethiopian and Western diplomatic sources in Addis Ababa concur that Somali Army units were engaged in the fighting here as recently as late September and early October.

This was one month after U.S. Under Secretary of State for African Affairs Richard Moose assured Congress that Somalia had decided to forego military means in pressing its longstanding objective of incorporating the Ogaden into a "Greater Somalia."

The Somali assurance were a key element in the Carter administration's defense of a deal to provide Somalia with $40 million in military aid in the next two years in return for access to the Indian Ocean's strategically located ports and airfields for the new American Rapid Deployment Force.

In the latest reported large-scale engagement, at least six "fully mechanized units" of battalion size took part in a two-pronged Somali incursion into Ethiopia's Bale Province west of the Ogaden starting Sept. 18, according to the official Ethiopian report of the incident.

Heavy armor, artillery, infantry and combat aircraft of both countries were involved in the battle which occurred around Yet and Dunsi on the southwestern border between the two countries and as far as 185 miles inside Ethiopia, a government communique said.

The invading Somali force was finally defeated Oct. 4 after suffering 1,100 casualities and the "annihilation" of the 275th Battalion, it said. Subsequently, the Ethiopians identified and published in the local press the tag numbers of six regular Somali Army soldiers captured in the fighting who belonged to the 184th, 183rd, and 9th battalions.

Western sources say there are probably still 500 to 1,000 Somali regulars still operating inside Ethiopia today, primarily in the eastern salient of the Ogaden jutting into northern Somalia.

This was the second time in five months the Ethiopians reported a major engagement with the regular Somali Army here. In early August, a government communique reported that 14,000 Somali troops had attempted in the previous two months to take the garrison town of Warder, east of here, and been beaten back suffering more than 3,000 casualties.

Western diplomatic sources say the size of the Somali force was more like 6,000 to 8,000 but do not dispute the Ethiopian claim that the Somali Army was involved in the battle for Warder, where the Ethiopian-supported Somali opposition organization known as the Somali Salvation Front is said to be headquartered.

The biggest difference in the latest confrontation between regular Somali and Ethiopian forces has been its closeness to the disputed border, one indication of the Ethiopian success in finally pushing the Somalis out of the Ogaden. The Somali Army had established a kind of buffer zone along much of the common frontier that the Ethiopians now have penetrated, provoking a strong Somali reaction, according to one Western diplomatic account of the fighting.

The presence of Ethiopian troops so close to Somali territory and the probability that the fighting spilled across the border may have been the main reason Somali President Mohammed Siad Barre suddenly declared a state of emergency late last month.

One side effect of the fighting along the disputed frontier has been the raising of tensions between Somalia and Kenya as well. Kenyan police recently intercepted a Somali column passing through that country's Northeastern Province on its way into southern Ethiopia, an infiltration route previously used by the Somalis in an attempt to get behind Ethiopian lines.

The Kenyan government has imposed a dusk-to-dawn curfew there and launched a manhunt for the troublemakers, causing many local Somalis to flee across the border into Somalia.

Signs of the improvement in the overall Ethiopian position inside much of the Ogaden are noticeably here in Kebri Dahar, where until recently Western Somali Liberation Front guerrilla bands were a serious threat.

Little more than a dusty market town and watering hole, the town still bears the scars of war and the six months it spent under Somali Army occupation in 1977. The shell-shattered remains of an Italian-built administrative center stand in the center, and at the entrance to the military post sits the rusting hulk of a Somali T34 tank. On the outskirts, a platoon of Ethiopian soldiers sleepily guard the now-empty reservoir of the town's main water system destroyed by retreating Somali troops.

Townspeople say the Somalis stripped the houses of windows, doors and corrugated tin roofs when they left, taking most of Kebri Dahar's 15,000 to 18,000 inhabitants back to Somalia with them.

Now they are returning from the Ogaden bush and Somalia refugee camps in ever greater numbers, most of them destitute and seeking help from the Ethiopian government to get going again.

Their return is just one sign of the changing times here. Others include the generally relaxed atmosphere and fact that it now takes only four days for convoys to arrive here from Jijiga, several hundred miles to the north, instead of more than 90 days as was the case a few months ago.

The return to what passes for normality in the war-ravaged Ogaden poses the enormous problem of rehabilitating and resettling its destitute nomads and even town dwellers plus the reconstruction of destroyed property.

The Ethiopian Relief and Rehabilitation Commission estimates the combined loss of livestock and property at nearly $3.5 billion after nearly six years of war and drought. It has drawn up a $425 million program to begin rehabilitating roughly 2.4 million people affected throughout southeastern Ethiopia, 800,000 of them nomads once living here in the Ogaden.