The United States applauded Somalia today for its decision this weekend to expel thousands Soviet military advisers and blamed Cuba for complicating efforts to find a peaceful solution to the war now aging between Somalia and neighboring Ethiopia.

State Department spokesman Hodding Carter said there would be no change in the administration's policy of refusing to provide arms to Somalia, once Moscow's closest ally on the African continent. If so, deep questions remain about the fate of the east African nation, which had now cut its military ties to the Soviet Union without having secured any certain source of arms in the West.

Commenting on Somalia's break in diplomatic relations with Cuba and cancellation of the 1974 treaty of friendship and cooperation with the Soviet Union Carter said that Somalia is now in "a far better position to pursue a truly nonaligned foreign policy." He charge that the problems in the region, known as the Horn of Africa, were largely the result of the large quantities of arms that the Soviet Union had first supplied to Somalia and was now giving to Ethiopia.

Carter also pointed to the increase of Cuban military advisers in Ethiopia over the past weeks, estimating their number at around 400. Altogether, there are about 550 Cubans serving there, a jump of between 100 to 150 in the past several weeks, he daid.

Carter said the United Sates continued to express its concern to Cuban authorities about its spreading military involvement in the Ethiopia-Somalia war and elsewhere in Africa, most notably in Angola where there are now around 20,000 Cubans.

Despite U.S. applause for the Somali decision to turn away from the Soviet Union and Cuba, it does not appear that the Carter Adminstration is intent on rushing to replace these nations as Somalia's main backer and arms provider. Carter said the administration continued to believe that "African problems should be solved by Africans themselves," indicating no change in the U.S. policy of noninvolvement in the Somalia-Ethiopia war.

The two east African nations have been at war since mid-July over the disputed semi-desert Ogaden region that lies between them. Somali insurgents belongings to the Western Somalia Liberation Front and backed by the regular Somali army have taken control of the Ogaden and considerable additional Ethiopian territory to the west of it, claiming that the land is historically part of "greater Somalia."

The Somalis have failed to capture two key Ethiopian towns, Harrar and Dire Dawa, following the delivery of massive quantities of Soviet arms, including tanks and Mig jet fighters, to Ethiopia. So long as these two towns remain in Ethiopian hands, the Somali conquest of the Ogaden remains in doubt.

Since mid-summer, Somalia has been searching for arms in the West to replace its Soviet-supplied weapons. At first, the United States, France and Britain indicated that they were ready to provide Somalia with "defensive arms" but they later changed their minds because of the ongoing war.

Somalia is believed to have obtained some additional Soviet arms, possibly including tanks, from Syria and Iraq as well as small amounts of military supplies from Saudi Arabia and Iran.

So far as is known, the Somali government has not found any major source of arms supplies to replace the Soviet Union. Meanwhile, Somali insurgents have reached an apparent stalemate in the war and now face a counteroffensive by the increasingly will - armed Ethiopian forces that could spill over into Somalia's territory.

A fear of just such a counteroffensive has led the Somalis to step up their search in the West for some key items, specifically anti - tank missiles and anti - aircraft weapons.

Somali diplomatic sources have suggested recently that if the United States felt it could not give arms directly, it might help arrange for the delivery of arms form one of its European allies. These sources suggested that West Germany might be the logical conduit and that its aid would be regarded as the quid pro quo for Somali cooperation in ending the hijacking last month of a Luthansa airliner.

The plane landed in the Somali capital of Mogadishu where it was later stormed by a special West German commando unit that killed all but one of the hijackers and rescued all the hostages.

It was not immediately known whether West Germany is prepared to provide Somalia with arms.

U.S. diplomatic sources said recently that if the West Germans or the French wished to provide Somalia with arms it was their business and the State Department would neither encourage nor discourage it.

Meanwhile, the United States has apparently decided to stay out of the intra-African conflict, just as it did in the fighting in Zaire's southwestern province of Shaba in March that threatened a direct confrontation between Zaire and its neighbor Angola.

Last year, Ethiopia ordered the United States to withdraw all its military adviser, just as Somalia has now told the Soviet Union. In addition, the Ethiopians closed down five American facilities, including the one - important radio communication relay station in Asmara.

Until that time, the United States was the main arms provider to Ethiopia in exchange for base rights dating back to 1953. Before breaking their military ties to Washington, the military officers ruling Ethiopia had lined up Soviet support to replace U.S. assistance.

Soviet tanks began arriving in Ethiopia in March and by mid-summer arms of all kinds were pouring into the country. Some estimates put the value of Soviet arms sent to Ethiopia at more than $500 million, while aid to Somalia between 1970 and 1977 went beyond $1 billion.

Precisely how long Somalia can hold out in face of concerted Ethiopian counteroffensive without military assistance from the West remains to be seen. The population of Somalia is only a little more than 3 million while that of Ethiopia is around 30 million. In addition, Ethiopia now has well over 200,000 men under arms, four to five times the size of the Somali army.