In the latest chapter of an on-again, off-again African saga, the Carter administration has decided in principle to inaugurate a military supply relationship with Somalia.

The initial U.S. offer, which is linked to Washington's effort to obtain the regular use of military air-fields and port facilities in that country, is likely to concentrate on "nonlethal" equipment such as trucks, transport aircraft and radar, according to informed sources.

Military supply to Somalia has been among the most complicated and at times controversial foreign policy issues before the administration. President Carter decided in principle in July 1977 to sell "defensive arms" to Somalia in an effort to induce that country to be "our friend," but backed away less than a month later because Somali forces had invaded the disputed Ogaden region of neighboring Ethiopia.

The Somali invasion precipitated large-scale Soviet and Cuban intervention to support Ethiopia. This continues. In mid-1978 the United States edged again toward starting a military relationship with Somalia, but again backed off in keeping with a decision to seek political rather than military solutions to African problems.

At one point in 1978, a senior Somali official complained that "America changed its mind seven times" about military assistance. With a reported 15,000 Cuban troops, several thousand Soviet advisers and communist bloc military supplies helping its age-old rival, Ethiopia, the regime in Somalia has never given up its quest for American weaponary and support.

Somalia's leverage has increased greatly now that the United States is seeking to use the port of Mogadishu and former Soviet facilities at Berbera to support the expanded U.S. military presence in the area. Two U.S. missions have visited Somalia since mid-December, in pursuit of these arrangements. A third U.S. mission, headed by Reginald Bartholemew, the State Department's director of politico-military affairs, left late last week for Somalia and other Indian Ocean areas were the United States is seeking more military access.

In the past six-months, fighting between Somali-backed forces and Cuban-Ethiopian forces in the Ogaden is reported to have increased gradually. The past two months have seen a greater use of Somali regular forces in the area, according to official sources.

The continuing conflict places the United States in an uncomfortable position regarding a military supply but high officials are reported to have decided that the need for U.S. access to Somali facilities is the overriding concern.

One source said Carter had made the basic decision to renew the offer of military supplies to Somalia.

A military program for Somalia will have to be accompanied by similar efforts with Kenya, U.S. officials conceded.The two countries have been in conflict in the past. The United States is also seeking expanded use of port and airfield facilities in Kenya.

In related developments:

Thirteen members of the House Foreign Affairs Committee, in a letter to Secretary of State Cyrus R. Vance, linked U.S. arms sales to Morocco with Moroccan diplomatic efforts to settle the war in the Western Sahara. But not objecting outright to the planned $232 million sale, the lawmakers virtually assured its approval by the House.

The United States had decided to sell refurbished A4 attack aircraft to Malaysia and Indonesia, according to official sources. The planes are presently in mothballs in an Air Force depot.