Somali insurgents fighting in the Ogaden region of Ethiopia appear to have taken over most of the area, comprising a third of Ethiopia's territory.

In the first interview granted to the Western press, the leaders of the Western Somali Liberation Front claimed that they intend to press home a campaign to finish off the Ethiopian military presence, now largely confined to major towns.

The little-noticed guerrilla war in the interior of the Horn of Africa could proved to be a more immediate threat to the hard-pressed Ethiopian government than the far more publicized secessionist war in Eritrea. Ethiopia's province fronting on the Red Sea.

The Somali government's apparent willingness not only to proclaim its longstanding territorial claim to the Ogaden, but also to let the guerrillas it backs there describe the military situation publicly, bears the risk of a direct clash between Somalia and Ethiopia.

The Soviet-trained and equipped Somali army is generally rated as the best-armed in black Africa. A clash between Ethiopia and Somalia could pose a major problem for the Soviet Union, which has been trying to establish a predominant influence in Ethiopia while attempting to maintain its close ties with Somalia.

Largely because of Ethiopia's new ties with Moscow, Somalia has been toying with making an overture to the West. In addition to strong backing from Somalia, the Ogaden rebels are reported to have recently received a large quantity of arms from Saudi Arabia, Washington's most important friend in the Arab world.

Leaders of the insurgency here said that the "total liberation" of the region is possible before the end of this year.

Western diplomatic sources here and in the Ethiopian Capital confirm that the situation for the Ethiopian military government in the Ogaden is extremely serious. The sources said that the peasant army being gathered in Addis Ababa to fight Eritrean guerrillas in the north would first be sent to the Ogaden to repel the invading Somali force of 3,000 to 6,000 insurgents.

So far, Somalia has not said very much in public about the Ogaden fighting. But the Ethiopian media have been reporting over the past several months the death of hundreds of "counterrevolutionary" bandits in the region where the Somali insurgents are operating.

Radio Ethiopia charge May 14 that Somalia and other unnamed countries in the region were looking for an excuse to launch a "direct aggression" against Ethiopia and accused Mogadishu of paying "hired agents" to fight in the Ogaden. It Claimed that there is a plot to deprive Ethiopia of all access to the sea and to weaken it by annexing the Ogaden.

The Ethiopian millitary government also faces a strong challenge to its authority in the northwest from anti-Marxist forces.

The Ogaden, a vast and mostly semi-desert region about half the size of Somalia, is inhabited by Somali-speaking nomads. It has been part of Ethiopia since its conquest in the late 19th century. Somalia has claimed the territory as part of "Greater Somaliland," incorporating all Somali-speaking peoples in the Horn since 1960, when Mogadishu became independent from Italian and British colonial rule.

Ethiopian authorities began expressing alarm about the deteriorating security situation in the Ogaden privately more than a year ago when Somali insurgents began stepping up their infiltration. The fighting took a distinct turn for the worst last fall, when the number of infiltrators increased dramatically just as the Ethiopian army was forced to withdraw many units stationed there to send them to fight in Eritrea and northwestern Ethiopia.

In his first interview with a Western correspondent, Abdulahi Hassan Mohamud, 37, Somali leader of the Ogaden insurgency, said his forces had taken control of seven towns in the region and destroyed six Ethiopian army battalions, about 3,000 soldiers, from September through April.

He also said that 11 Ethiopian tanks and 16 armored personnel carriers - all U.S. equipment - had been put out of action and 12 senior Ethiopian officers killed by his forces in the past nine months. In three of the seven cities, so far "liberated" - Imi, El Kere and Degahabur - the Ethiopian army is still holding out in isolated garrisions while the insurgents control everything else in town, he said.

The liberation front captured an Ethiopian army DC-3 on the ground carrying a colonel, a major and a lieutenant, who are prisoners, Abdulahi said.

He also said his forces had captured a United Nations helicopter after it had landed in a village controlled by them. The aircraft was on an "espionage flight," he said. Its two white occupants were released "for humanitarian reasons," he said . The helicopter's Ethiopian occupants are still being held, he added.

Five other foreigners taken prisoner in March, including an American, had also been freed on similar humanitarian grounds, Abdulahi said.

He also said that the front had attacked the U.S.-built radar station in Jijiga but had failed so far to destroy it.

Most of his claims, including the destruction of six Ethiopian army battalions, were generally vertified by Westered diplomatic sources here or in Addis Ababa.

Abdulahi is secretary general and military commander of the Front for the Liberation of the Abyssinian-Occupied Somali Territory, also known as the Western Somali Liberation Front. It was founded in 1963, when the Organization of African Unity was established, to press the Somali claim to the Ogaden in various diplomatic forums.

The 2 1/2-hour interview with him and two other front leaders took place here in Mogadishu, where the organization has its headquarters. In the past, the front had shied away from all publicity except an occasional interview in the Arab press.

Abdulahi said his front is fighting in four eastern and southern Ethiopian provinces - Harrarghe, Bale, Sidamo and Arussi - and estimated that it is facing an Ethiopian force of around 20,000 men, mostly local militiamen and frontier police. He said the Ethiopian army's 3d Division, based in Harrar, has been almost "completely destroyed with its remaining units bottled up in small towns throughout the four provinces in isolated fixed positions.

"Like Eritrea," he said, "here, too, we have liberated all the territory except the major towns." In Eritrea, the separatist Eritrean Liberation Front has won control of practically all the territory and all but a few towns and cities.

The front leader said that Ethiopian troops had been on the offensive in the Ogaden from September through February but that, starting in March, they had changed their tactics and gone one the defensive after sustaining heavy casualties.

Abdulahi would not say how large a force he commands, but Arab sources close to his front said it claims to have 6,000 to 8,000 insurgents in the Ogaden. Other sources here said, however, that they consider this figure exaggerated and the number of insurgents fighting inside Ethiopia is 3,000 to 6,000.

Abdulahi told in detail of two major engagements.

The first and larger, he said, took place Feb. 11 about 25 miles south of Jijiga, in Harrarghe Province, and resulted in the death of 300 Ethiopian soldiers and the suicide of the battalion commander. He said armored personnel carriers and four tanks were destroyed, while 14 mortars, 200 M-14 rifles and 5 million rounds of ammuniction were captured.

He put the front's losses at 26 killed and 40 wounded in the February battle.

The second engagement, May 1, involved an ambush by his forces of 260 Ethiopian armored cars, personnel carriers and trucks just south of Degahabur on the road from Harrar to Kebri Dehar in central Ogaden. The front leader said that in six days of "uninterrupted fighting" the Ethiopians had lost "not less than 200 soldiers" and 32 vehicles, including four armored personnel carriers.

He did not indicate what the front's losses in the second battle were, nor was any independent confirmation of his account of the two engagements available here.