Despite continuing expressions of anger and frustration from the United States, the Kremlin is pressing on with its efforts to bolster its embattled ally, Ethiopia, in the strategic Horn of Africa.

The Soviet leadership met with Cuban Defense Minister Raul Castro and South Yemeni Prime Minister Ali Nasir Muhammad this past week in sessions Western this past week in sessions Western observers here interpreted as discussions to further coordinate the flow of arms and advisers to Ethiopia.

South Yemen is the principal staging area for supply flights into Ethiopia, just across the Red Sea. Ethiopia's Kremlin-backed radical Marxist government is battling insurgents on two fronts and contending with substantial civil opposition as well.

Cuba has supplied up to 2,000 military and technical advisers to Ethiopia and the Soviets about 1,000. U.S. intelligence sources were said to have reported that Castro had just come from Ethiopia.

As if to dramatize the importance the leadership ascribes to its efforts on behalf of Ethiopia, Soviet President Leonid Brezhnev, who had not appeared in public since early January, reappeared to confer with both men.

The Soviets and Yemeni leader Ali Nasir have pronounced themselves well staisfied with their talks. Tass, the official government news agency, said that Nasir "noted a coincidence of views of the two countries on matters relating to the present international situation."

At the same time, this kind of carefully orchestrated public display for South Yemen is sure to add to the disagreeable air that has now settled over U.S.-Soviet relations. The new tone comes even as the two superpowers are seeking ways to benefit themselves and each other through agreements on arms control, nuclear non-proliferation and a host of other major issues.

One lell-informed Western diplomatic source sought to make this point unmistakably clear to a listener. "This kind of adventuring imposes a price on Soviet relations," he said. "There has been a cooling off since the beginning of the Soviet airlift" some weeks ago.

The Soviet-American flareup over the Horn of Africa relates in part to the desire of both nations to exert control in their own interests beyond their borders. But the specific concern of the moment is access to control points along the vital Red Sea and Indian Ocean navigation lanes, where the giant tankers ply their trade between producer and consumer nations.

Ali Nasir, the Yemeni leader, at a state dinner with his Kremlin hosts Thursday, emphasized this concern. "We have always stressed the need for removing imperialist military bases in the Indian Ocean. This is a pre-condition of any conversation on attaining stability in the Indian Ocean and on cooperation of the peoples living in that region." He referred specifically to the U.S. base on Diego Garcia in the Indian Ocean.

There is nothing mysterious about this grappling for power and influence. The rationale can be found codified in the teachings of such 19the Century strategic theorists as Alfred Thayer Mahan and von Clausewitz.

In the view of some observers here, these straightforward strategic considerations will compel the Soviets to try to replace their Red Sea ports lost when Somali ousted the Soviets recently. In addition, a stronger Soviet presence in South Yemen can serve as a rebuff and possible check to neighboring Saudi Arabia, whose conservative government has tacitly supported Somalia in its war of insurgency against Ethiopia over the Ogaden region.

One source, however, asserted that the Kremlin's path is not without potential pitfalls. He recalled that Somalia began its invasion of Ethiopia with Soviet arms. "The Soviets say they tried to keep the Somalis at home, but at the same time, they gave the Somalis enough weapons to head for Ethiopia and now the Ethiopians may find themselves in the same position."

At present, this person pointed out, the Soviets are getting some support from other African nations whose governments preside over complex multi-tribal, multi-national populations.

These leaders fear that a Somali success in the Ogaden may spell a wave of similar invasions elsewhere on the continent where national boundaries frequently cross or divide ancient tribal or old national areas.

But if Ethiopia counterattacks and dislodges the Somalis from the Ogaden, which some here believe is possible, can the Soviets prevail upon their new allies to go no farther than tha original Ethiopian-Somali frontier?

While it is sheer speculation and seemingly wildly improbable in view of the weekend condition of the embattled Ethiopian leadership, the tussle for control of the Horn of Africa clearly contains more surprises.