Secretary of State Alexander M. Haig Jr. held an unannounced meeting with a senior African diplomat here today to discuss U.S. support for peace-keeping efforts in Chad, another phase of the administration's growing anti-Libya program.

Haig's meeting with Nigerian Foreign Minister Ishaya S. Audu concerned that country's request for U.S. aid to underwrite the dispatch of 2,000 troops to replace Libyan forces in strife-torn Chad. According to U.S. sources, Nigeria has been the leading backer of the all-African peace-keeping force authorized by the Organization of African Unity. A Nigerian general is to head the force and Nigerian troops are expected to fill most of its ranks.

The administration has informed Congress of plans to allocate about $12 million for support of the OAU effort, principally for Nigeria and Zaire, another major contributor of troops. U.S. Air Force transport planes began flying the first 53 tons of support items, including Coca-Cola, tents and medical supplies, to Zaire within the past several days.

State Department sources said the size and nature of the U.S. aid for the Nigerian part of the effort has not yet been determined. This is reported to have been a major topic of today's meeting, which was kept quiet at the request of Audu.

Nigeria is among the friendly nations briefed by the United States in recent days about the evidence Washington claims to have of Libyan assassination plots against U.S. officials at home and overseas. The evidence has not been publicly disclosed, although a State Department summary of it is said to refer to 25 reports of Libyan plotting. While refusing to discuss it publicly, U.S. officials have continued to insist the evidence is convincing.

French Foreign Minister Claude Cheysson, who heard Haig's description of it in a NATO session Thursday, seemed unimpressed. Asked by French reporters about the impact of Haig's recitation, Cheysson replied that Haig "convinced himself."

Partly because of French satisfaction with the Libyan withdrawal from Chad, France's policy on Libya seems to be going in the opposite direction from that of the United States. The French Foreign Ministry announced yesterday a decision to return progressively to normal relations with Libya because of a belief that "the attitude of the Libyan government is no longer one of external destabilization."

Haig, meeting reporters today, said France's decision to upgrade its relations with Tripoli is not a rejection of the U.S. policy on Libya. The secretary of state called the French decision "an extension of French foreign policy under President Francois Mitterrand" and suggested that it was linked to the Chadian situation where French diplomacy has been "very active with substantial coordination with the United States."

Nigeria, which borders on Chad, has a potential interest in the bilateral and multinational aspects of U.S. policy toward Libya. If Washington should follow the withdrawal of Americans from Libya with a decision to boycott Libyan petroleum, Nigeria could be an alternate supplier of the same type of African light sweet crude oil that U.S. refineries now obtain from Libya. In this case, Nigeria, which recently suffered a decline in its oil sales and a resulting financial pinch, would stand to benefit. A shift to Nigerian supplies would also assist the U.S. refineries using this type of African petroleum, especially if Nigeria can raise its productive capacity.

Details of the oil-related part of the Haig-Audu meeting were not available. A U.S. official, tacitly acknowledging that oil was discussed, said the meeting covered every facet of Nigerian-American relations.

Today was the last day of Haig's four-day stay in Brussels, which centered on the semiannual NATO foreign ministers' meeting. On Sunday, he is scheduled to stop in Israel, then will visit Turkey, Pakistan, India and Egypt late in the week.