The Carter administration is preparing to ask Congress for $90 million to $100 million in military and economic assistance to Kenya, Oman and Somalia in return for expanded access to ports and airfields in those countries for U.S. naval and air units, according to informed sources.

The three countries, all bordering on the Indian Ocean, have expressed a willingness to provide such access and the United States is in the process of negotiating final agreements with them.

The administration wants to gain at least some foothold in the region which would make it easier for the United States to sustain military forces near the oil-rich Persian Gulf.

The aid package will be spread over a two-year period covering fiscal 1980 and 1981 and will include money to finance credit purchases of military equipment, economic support funds and some development and food aid, sources say.

The funds are part of the administration's overall security assistance budget.

It would be the first time any of the three countries has received aid under the economic support part of that budget.

This is significant because money from this fund reportedly has fewer restrictions on how it can be used than other forms of aid. Thus it is politically more useful to both the United States, in terms of influence, and the recipient government, in terms of flexibility.

Though the final amounts remain to be worked out, sources say the total aid package to Kenya is the smallest of the three, with Oman and Somalia roughly comparable.

Kenya has had small amounts of U.S. credit in the past for purchase of military equipment and that will now be expanded. Oman and Somalia, however, have not had military financing before, at least in recent years, officials say, and both will now receive it.

Of the three countries, the aid to Somalia is by far the most controversial because of that country's continuing feud in the disputed Ogaden desert with the Soviet-backed regime in neighboring Ethiopia.

The new aid package reportedly allows credits only for purchases of so-called "non-lethal" military equipment, such as trucks and radar sets, for Somalia.

The Somali government is said to want a lot more military aid from the United States than is currently on the list, but sources say there is no indication that the deal is threatened by the currently planned level of U.S. assistance.

Critics of the involvement with Somalia -- including some in government and Congress -- fear it could drag the United States into an African war and be seen as supporting a government that is alto unpopular elsewhere in Africa.

The Carter administration, however, fearful of Soviet inroads in the region, believes that U.S. strategic needs outweigh the potential problems with Somalia.