The Reagan administration has told African nations that it is willing to lead the way toward a negotiated settlement of the Namibian conflict but will abandon the effort unless there are realistic prospects of success.

This message was taken to 12 African countries earlier this month by Chester ya. Crocker, who has been nominated to be assistant secretary of state for African affairs, according to a State Department briefing for reporters yesterday.

The possibility that the Reagan administration might "disengage" from an unproductive international effort on Namibia was "not a threat but a reality," said a State Department official intimately familiar with the Crocker trip. He cannot be named under the ground rules of the briefing.

The new U.S. administration has "a limited and finite amount of capital" to expend on foreign affairs and therefore will not continue major efforts in an area likely to produce failure, Crocker is said to have told the Africans.

In the case of Namibia, the thinly populated but mineral-rich land between South Africa and Angola, the administration sees three choices:

A fruitless attempt to pursue a settlement under the original terms of U.N. Security Council resolution 435, which continues to be strongly backed by African "front-line states" but has been "something of a dead letter" for at least a year, in the view of the Reagan administration.

A "strengthened and reinforced" resolution 435 involving formal constitutional protections and implementation guarantees for groups within Namibia before an election to choose an independent government there.

This approach would be facilitated by an improved "regional climate" flowing from the withdrawal of Cuban and Soviet forces from Angola, in the administration view.

U.S. disengagement from the Namibia effort if it cannot find a way to work productively with black African states, South Africa and the four other Western nations -- Britain, France, West Germany and Canada -- that have comprised the "contact group" seeking to arrange a settlement.

The next step toward a solution, after Security Council proceedings this week in New York is a scheduled meeting Sunday in Rome of foreign ministers of the "contact group" nations, including Secretary of State Alexander M. Haig Jr.

After that, the United States expects an airing of the issues with South African Foreign Minister Roelof (Pik) Botha, who is scheduled to visit Washington May 15, and further detailed discussions by senior officials of the contact group nations who are to gather here in the second half of May. oCrocker, in his recent visit to Angola, told the Luanda government that the United States will not establish diplomatic relations without a withdrawal of Cuban troops, according to the State Department official.

Crocker was not asked for, nor did he give, assurances that the administration will not support UNITA, an Angolan guerrilla faction led by Jonas Savimbi, who opposes the Luanda government, the State Department official said.

The official said, "There have been no decisions taken to become involved in direct support of Savimbi, nor is there any plan to do so in terms of tangible support."

However, the official called Savimbi "a genuine and legitimate nationalist leader" and said it is "a simple political fact" that "there will be no peace in Angola until [Savimibi's] people have a cut of the pie."

How a political compromise between the Angolan government and Savimbi might be arranged is "not for us to say," the U.S. official said.