The ranking members of the House subcommittee on Africa yesterday released a letter cautioning that the shipment of military supplies to aid Somalia in the latest round of fighting in the volatile Horn of Africa could harm U.S. interests in the region.

Reps. Howard Wolpe (D-Mich.), subcommittee chairman, and William Goodling (R-Pa.), the ranking minority member, asked Secretary of State George P. Shultz to broaden the U.S. response to the fighting by developing a high-level diplomatic initiative seeking a cease-fire. Somali troops are fighting a force near the Ethiopian border that includes Somali guerrillas and, according to Somali President Mohammed Siad Barre, Soviet- and Cuban-backed Ethiopian troops.

The congressmen warned that "a mainly military U.S. response . . . would probably be inadequate and might enhance the risk of a perceived U.S. humiliation in Somalia." They also cautioned that the current conflict could raise tensions between the United States and the Soviet Union.

The State Department announced Saturday that the supplies were being airlifted to Somalia because of "incursions by Ethiopia and Ethiopian-supported forces." The first part of the shipment arrived Sunday.

Ethiopia has denied that its troops invaded the Somalian border areas, and yesterday it formally protested the military airlift in a letter handed over to the U.S. charge d'affaires in Addis Ababa.

The United States has been working to improve relations with Somalia--which is considered a strategic point for defense of the Persian Gulf--since the African nation expelled Soviet advisers in 1977. The two countries signed an agreement in 1980 allowing the United States access to ports and airfields if necessary in a military emergency.

Egypt, which has supplied Mogadishu with arms including tanks, was believed to have begun sending antiaircraft defense materiel to Somalia, Agence France-Press reported.

Although they condemned Ethiopia's "current actions in seizing Somali territory and militarily supporting" the insurgent Somalia Democratic Salvation Front, the congressmen said that limitations of U.S. military resources at this time and Ethiopia's "overwhelming military predominance in the region" would mean that a mainly U.S. military response "would probably be inadequate."

The letter also asks the Reagan administration to inform Somalia that continued arms aid is jeopardized by reports of Somali interference in Ethiopia's Ogaden region. It asked the administration to consider reports of serious human rights abuses by both governments.

Wolpe, in a telephone interview yesterday, said he hoped the administration would broaden its policy in the region and move away from the strictly "bilateral way" it handled relations there. He suggested that both nations have hinted that they are interested in establishing some sort of "new kind of political order" with each other and that the United States should begin to work with those concepts.

U.S. officials have refused to detail specifically what is being sent to the Somalis. It includes small arms, ammunition, air defense equipment and transport, communications and engineering supplies, a Pentagon official said.