Mombasa, Kenya — The United States is gaining a firmer military toehold in East Africa. One sign is the steady improvements being made at US expense on port and air facilities in this steamy Indian Ocean port, where US ships regularly call. Another is the access the US gained in 1980 to the sheltered port and the former Soviet air base at Berbera farther to the north in Somalia.
Both developments sprang from a sudden and rather frantic recognition by US defense planners in late 1979 that not enough attention was being paid to the strategic importance of the Indian Ocean.
The revolution in Iran and the Soviet invasion of Afghanistan underscored the need for greater US presence in the region to guard Western oil suppliers in the Gulf. Thus the US developed its Rapid Deployment Force and gained access in 1980 to airfields and ports in Oman.
That stepped-up presence in the Indian Ocean has been achieved. A US diplomat in Kenya says: ''In the late 1970s we were not active (in the Indian Ocean). Now we are.''
Still, political uncertainties in East Africa make the military toehold gained over the past three years a potentially slippery one, diplomatic sources point out. President Muhammad Siad Barre's government in Somalia has faced mounting attacks from rebel forces based in Ethiopia. And last year's coup bid in Kenya shook what seemed to be a rock of stability in East Africa.
The US continues to rely principally on Diego Garcia as its main military foothold in the Indian Ocean. The United States has a base on the island, and military hardware is stored there.
The agreements the US has signed with both Kenya and Somalia only grant US forces ''access'' to those countries' port and air facilities. Precisely what that term means is unclear because the agreements are classified.
In Kenya, US officials heatedly deny there is any intention to turn Mombasa into a full-fledged base. Mombasa is described as principally a ''rest and relaxation'' port for the US Navy.
But the US is busy making improvements in Mombasa, some intended to facilitate American military use of this coastal town, should the need arise.
Already the harbor on one side of Mombasa has been deepened and widened so US warships, particularly aircraft carriers, can come right up to the port instead of having to dock out at sea. It is explained as a step to make ''liberty easier'' for American crewmen.
But the harbor dredging seems in line with other efforts to make Mombasa more efficient, should the US ever need the port as a resupply depot.
Besides the harbor, the local airport is being upgraded. Bulldozers are extending the airport's short taxi strip to stretch the full length of the main runway. Should the United States need to airlift supplies into Mombasa, the present work would increase the speed with which deliveries could be made, US sources point out. Also, the US is funding the installation of new navigation equipment at the Mombasa airport as well as improving the road to the Kenyan naval base here.
All of this activity in Mombasa will cost the US about $50 million.
There is plenty of speculation in Kenya that the US is already storing nonmilitary goods, such as food and water, in Mombasa for use should troops have to be sent in quickly to the Middle East. US diplomatic sources deny this. They insist that there is no intention of storing anything in Mombasa.
But the importance of Mombasa as a potential supply point is underscored by the fact that equipment could be flown in here from the US many hours quicker than they could into Diego Garcia.
Still, talk of any military role for Mombasa is sensitive and avoided as much as possible in Kenya. The country, while viewed in practice as pro-West, is officially nonaligned. Too overt a US military presence in Mombasa could embarrass the Kenyan government.
However, many Kenyans welcome use of Mombasa as a liberty port. Last year some 31 US ships docked here, putting ashore some 25,000 men and women. It is estimated they pumped some $10 million into the Kenyan economy.