The United States has decided to delay sending a military mission to Somalia following a strong public warning from Ethiopia that it will sever diplomatic relations if Washington goes ahead with its plan to provide the Somalis with arms.

Diplomatic sources here say the Carter administration is now reconsidering its decision to send $15 million in "defensive arms" to Somalia in light of the Ethiopian threat. The visit of the military mission was in providing the arms.

Complicating the American dilemma over how to balance its relations with the two neighboring enemy countries in the strategic Horn of Africa is mounting evidence that Somali guerrillas are again escalating their war in the hotly disputed Ogaden region of southeastern Ethiopia.

Here at the just concluded Organization of African Unity summit meeting. Ethiopia and Somalia made it clear that their dispute is far from resolved and that another war is a distinct possibility.

Just how Washington will reconcile its arms offer to Somalia with the Ethiopian threat to break diplomatic relations remains to be seen. But right now, it is alienating Ethiopia by having made the offer and Somalia by not coming through with the arms.

But Washington is under heavy pressure, particulary from Saudi Arabia, to do more for Somalia, including the provision of some arms to help compensate for the loss of Soviet military assistance. Last November, Somalia cut all its military ties with Moscow and broke diplomatic relations with Havana following the Soviet-Cuban decision to back Ethiopia in the Ogaden dispute.

Soviet arms and Cuban combat troops proved decisive in the Ethiopian victory last winter over Somali guerrillas, backed by the regular Somali army, which seized control of virtually the entire Ogaden region. The semi-desert area is inhabited by Somali-speaking people and the Somali government refers to it as "Western Somalia."

Somalia and Ethiopia have delivered long, bitter attacks against one another at the African summit talks and an attempt by the OAU to reconcile the two has failed.

Somali President Mohammed Siad Barre accused Ethiopia of amassing troops along the Somali border in preparation "an all-out invasion" of his country and bitterly attacked Cuba for sending "mercenary forces" to bolster Ethiopia.

Calling Cuba the "surrogate" of the Soviet Union and denouncing the close alliance between the two, the Somali leader called for "collective African action" against the "Cuban menace" and Cuba's exclusion from the nonaligned movement.

"Cuba is surely unworthy of its membership in a movement dedicated to the preservation and safeguarding of the principles and policies of non-participation in collective alliances which include the big powers," said Siad Barre.

Warning that there is little prospect for peace in the region unless the Ogaden issue is settled, the Somalia president listed five conditions for a settlement, including the withdrawal of all foreign troops from the Ogaden and the right of self-determination for the people living there.

Ethiopia's foreign minister, Feleke Gedle-Ghiorgis, replying to the Somali charges, said the Somalis were continuing their "war of aggression" and had recently twice attacked trains on the vital Addis Ababa-Djibouti rail line. He said Somali regular troops were operating once again inside the Ogaden and asked, "For how long is Ethiopia expected to exercise restraint?"

Calling Siad Barre's statement "tantamount to a declaration of war," Feleke listed Ethiopia's four conditions for a peaceful settlement of the Ogaden dispute including Somalia's unconditional renunciation of territorial claims on Djibout and parts of Ethiopia and Kenya, and acceptance of the OAU's principles of respecting colonial borders and not using force in interstate affairs.

While Somalia has not altogether renounced its aspirations to regain its "Lost territory" in Ethiopia, Siad Barre made the clearest statement to date before the African summit here that "we have no claim on the territory of any state." He added however, that Somalia remained committed to supporting the struggle of peoples living under "colonial rule." It regards the Somali-speaking nomads of the Ogaden and the Eritreans in Ethiopia's northernmost province as being in this category.

The United States has been insisting on a Somali promise to respect its neighbors borders before Washington will provide any arms. Somalia is reported to have given more or less such an assurance in private, although it has said nothing to this effect in public.

Having given the required promise, Somalia is now awaiting with growing impatience the "defensive arms" Washington has announced it is now ready to provide.