Libyan troops have completed their pullout from the Chadian capital of Ndjamena and are being replaced by Zairian units, the first contingent of an African peace force whose task appears increasingly difficult, according to reports reaching Paris.

Confusion and delay surrounding efforts by the Organization of African Unity to get the force into place underlined the precariousness of its mission in a country rent by a decade-old civil war. The fighting flared up as the Libyans flew home.

The United States and France have both proclaimed willingness to help the stricken central African country, seeking to demonstrate to President Goukouni Oueddei that he can get along without Libya's Muammar Qaddafi. But now that Qaddafi has recalled his troops, neither Washington nor Paris -- nor even the OAU countries -- seem eager to become deeply involved in Chad.

France has for the last month been urging the OAU to dispatch peace-keeping troops swiftly to back Goukouni and has promised "financial and logistical help." The United States has endorsed the appeal as a way to replace the 8,000 Libyan soldiers who had been in Chad and to prevent Qaddafi from consolidating his influence there.

At the same time, French diplomatic sources say, Paris and Washington so far are unwilling to shoulder the entire bill for the OAU force. As a result, the six African countries that have promised to contribute troops -- Nigeria, Zaire, Senegal, Togo, Guinea and Kenya -- have been unable to agree on financing, they add.

In addition, the rebel forces of former defense minister Hissene Habre have, according to reports from French correspondents in Chad, captured at least two towns in new fighting in the eastern part of the country.

Habre's representative here said in a statement today that OAU support for Goukouni is unacceptable and strongly implied that Habre's Sudanese and Egyptian-supplied guerrillas will oppose the peace forces.

An 18-man team of African officers arrived in Ndjamena today to assess the force's logistical needs, Agence France-Presse said. The officers are to report in the next few days to African officials due to meet in Zaire, French diplomats said.

It was not clear whether presidents Jaafar Nimeri of Sudan and Hosni Mubarak of Egypt actively continue to support Habre in defiance of the OAU, to which both belong. In any case, Habre's guerrillas have received enough Egyptian arms through Sudan to make them a potent force by Chadian standards.

The United States tacitly approved the arms supplies to Habre as a way to harass Libya before accepting France's appeal for an OAU force to strengthen Goukouni, French sources say.