The Soviet Union is urgently pressing Somalia for an immediate crease-fire in the fighting in southeastern Ethiopia between Somali insurgents and the hard-pressed Ethiopian army, according to Arab and Western diplomatic sources here.

Moscow is seeking to save the pro-Soviet regime of Lt. Col. Mengistu Haile Mariam in Addis Addis from being overthrown or simply collapsing following its multiple military defeats both in its northern province of Eritrea and the disputed Ogaden region along the Somali border, according to these sources.

The Soviets are thought here to be proposing to its two northeast African friends a division of the disputed territory that would give Somalia possession of virtually all of the Ogaden region but keep part, including the two key towns of Harrar and Dire Dawa, as part of Ethiopia.

There is no indication yet, however, that Somali President Mohamed Siad Barre is yieding to Soviet pressure. On the contrary, it appears that the Somali insurgents of the Western Somalia Liberation Front have the full support of the Somali government to complete as quickly as possible their conquest of all the disputed territory before the big powers or other Africa countries can act to prevent it.

Ethiopia has called upon both the organization of African Unity and the United Nations to take action to halt the fighting, and a special mediating committee of 10 African countires was scheduled to begin its work today or Thursday in Libreville, Gabon, with delegations from both Somalia and Ethiopia present.

A spokesman for the Western Somalia Liberation Front said here today that Somalia had put no pressure on the front to slow down the pace of the fighting. "We do not expect any pressure and even if they do this, we will not accept it," he said.

"Our land extends up to the Awash River and we are not going to compromise. We are going to take back all our land," the spokesman said in an interview at the Front's headquarters here. "Under no circumstances can we stop before taking Harrar and Dire Dawa."

Harrar in particular is a prime target of the Somali insurgents because it is an old Islamic religious center whose main mosque was converted into a Coptic church after Emperor Menelik conquered the town and incorporated it into the Ethiopian empire in 1887.

The Front spokesman said today that Somali territorial claims covered all of Ethiopia's four southeastern provinces of Bale, Sidamo, Arrusi and Harrarghe and were not limited to just the Ogaden region itself. He gave as the two most western points Awash Station on the Awash River, about 140 miles east of Addis Ababa, and Asela, 110 miles to the south.

Altogether, the disputed territory represents about a third of the land mass of Ethiopia and it equivalent ot half that of present-day Somalia. The population of this country, now around 3 million, would more than ple, according to the Front's caculations.

The Soviet Union is understood to have broached the idea of a compromise between the two neighboring Marxist states more recently in Moscow this past week. Ethiopian and Somali delegation were present in the Soviet capital then and may still be there.

The Soviets appear to be undertaking the difficult task of reconciling the sharply conflicting nationalist interests of Somalia and Ethiopia, without losing their position in either. To achieve this, they must maintain Ethiopia's Col. Mengistu in power a feat that is likely to prove impossible if he is held responsible for the loss of all the vast Ethipian territory claimed by Somalia.

Ethiopia claims that it is the victim of a full-scale invasion by regular Somali army units. The Somali government and the Front vigorously deny this and maintain that only the front's 30,000-man guerrilla force is fighting inside Ethiopia.

To date, the goals and territorial claims of the Front and the Somali government have been virtually identical. Whether Present Siad would be willing to settle for less than the entire territory to which Somalia has laid claim since it s independence in 1960 remains to be seen, but he has never before indicated any flexibility on his government's position regarding the regions inhabited by Somali-speaking peoples inside Ethiopia.

Nonetheless, Arab and Western dipomats here do no exclude the possibility that President Siad may come to view a compromise that is also acceptable to Ethiopia as preferable to the long struggle necessary to hold such a vast region against the will and army of his much larger neighbor.