The United States will airlift the remaining French Legionnaires out of Zaire over the next 24 hours and plans to fly in the first all-African peacekeeping force the French are trying to assemble, officials said last night.

The evacuation of the Legionnaires was described as a "definite" plan, presumably involving U.S. Air Force C130 and C141 transport planes, while the airlift of the African replacements was characterized as "tentative."

"The French are clearly taking the lead in this," said one source last night in describing the effort to assemble a force of Moroccans, Senegalese and other Africans to keep peace in Zaire.

If the French succeed in their effort to put together the desired force of African officers and troops, sources said, the United States stands ready to fly them into Zaire. While a hitch could still develop, officials said the effort was progressing well.

The United States' role in the evacuation and replacement operations would be strictly limited to airlift and supplies, not providing combat troops or advisers, U.S. officials stressed.

Belgium, Britain, France, the United States and West Germany are scheduled to meet in Paris on Monday to discuss ways to organize a pan-African force.

If such a force is assembled and flown into Zaire in the meantime, U.S. officials conceded last night, this would put a wholly different character on the upcoming meeting.

French Foreign Minister Louis de Guiringaud told The Washington Post on Monday that Presidents Carter and Valery Giscard d'Estaing of France had agreed the previous Friday to help African nations defend against "destabilizing external forces."

"We agreed," Guiringaud said of that meeting, "that if the African states put together some operative arrangement to defend themselves against those destabilizing external influences, we will help them and back them."

Although other leaders of NATO countries, notably British Prime Minister James Callaghan, reacted coolly toward the idea of putting together a peacekeeping force for Africa, the U.S. readiness to fly such a force into Zaire indicates Carter was not dissuaded by arguments raised during the recent two-day North Atlantic Treaty Organization summit here.

In contrast to the French, who hope to replace their Legionnaires with Africans in Zaire, Belgium announced yesterday that it was sending fresh troops to Zaire to replace its 600 para-troopers in Shaba Province.

Belgium Defense Minister Paul Vanden Boeynants said in an interview with the Dutch newspaper De Standard that while he endorsed the idea of a pan-African military force, it was impossible to predict when such a force would be deployed.

France sent about 800 Legionnaires to Zaire and Belgium 1,700 troopers. A battalion of about 600 Belgium paratroopers is still at Kamina air base north of Kolwezi.

Both the French and Belgian forces rushed into Zaire to save Europeans after the forces under Zaire President Mobutu Sese Seko could not hold out against attacking rebels.

Mobutu yesterday commuted to life imprisonment the death sentence of the brigadier general who commanded the garrison of Kolwezi which fled when the rebels attacked.

Rebel attacks persist in Zaire, according to sources there, but their effectiveness is difficult to ascertain. A spokesman for the Congolese National Movement said in Brussels yesterday.

The Manchester Guardian reported from Kinshasa that an unknown number of insurgents "is said to have crossed from southern Suda and burnt houses and crops in the town of Faradje, 40 miles from the border."

United Press International quoted diplomatic sources as saying that Mobutu's troops had been clashing in various parts of Kasai Province, while Belgian radio said the towns of Bunia and Aba in the province of Upper Zaire were held by rebels.

The pan-African force the French are taking the lead in assembling would consist of troops from Gambia, the Ivory Coast, Morocco and Senegal.