The Organization of African Unity's first multinational peace-keeping force, rushed here a month ago to separate factions in Chad's civil war, is in serious financial difficulties.

The arrival of Nigerian and Zairian troops in December checked the dramatic sweep through eastern Chad by the army of rebel leader Hissene Habre that appeared headed toward the capital of Ndjamena and an easy victory over the country's 10-faction coalition government.

However, the OAU peace-keeping force will soon be out of money. OAU officials, according to well-informed official African and diplomatic sources, are preparing an unprecedented request for funding by the United Nations. The United Nations has never financed a force that was not under its auspices.

With its intervention here, the OAU accepted a role its founders had envisioned at their first meeting in 1964. Since then several unsuccessful attempts have been made to create an all-African force that would be available to repel invasions of its weakest members, suppress inter-African clashes and push civil war factions to the negotiating table.

Time after time, however, the proposal for a peace-keeping force had been shelved because of the high cost.

After deciding to intervene in Chad, the scene of one of Africa's thorniest conflicts, the OAU received initial financial support from France and the United States. Both those countries, as well as numerous African governments, were anxious to see an OAU contingent replace the 7,000-plus Libyan soldiers here.

It is doubtful, however, that France or the United States will contribute anything like the estimated $80 million needed yearly to maintain the force.

Libyan leader Muammar Qaddafi pulled his troops out last November at the request of the Chad government. His intervention had been interpreted by many of Chad's neighbors as an attempt to carry out what they felt was his long-term goal of annexing the country. After the Libyans left, however, Habre was able to burst out of his sanctuaries in neighboring Sudan and launch an offensive against the central government.

Chad's president, Goukouni Oueddei, has bitterly complained that the OAU force is not fulfilling its role because it refuses to attack Habre's forces.

No evidence could be found here to substantiate persistent rumors in Ndjamena and radio reports from Libya claiming that Habre's forces have been filtering past Ati in small groups or had encircled Ati. An inspection of this village of 6,000 and a flight over the sandy flatlands around it revealed no sign that Ati had ever been surrounded by Habre's troops.

Turbaned horsemen rode in and out of the village seemingly oblivious of the Nigerian and Zairian troops. Ati's central market was in full operation in the open square, set among a casbah maze of flat-roofed, orange-tan mud houses.

The district administrator, Ahmat Mohamat Zenallah, said the villagers feel secure and claimed that Habre's forces had never approached the town. The area has suffered a 10-month drought, Zenallah said, leaving only the barest minimum for the peasants to eat, much less support a 4,000-man army marching 250 miles to Ndjamena.

A Zairian captain, who declined to be identified, said the OAU force, which is up to two-thirds of its authorized strength, does not check out either the villagers or the peasants coming in from the countryside.

"We can't tell the difference between the local population and Habre's men anyway," he said. Habre's troops "probably come into Ati in civilian dress to make a reconnaissance of our positions, but we want them to know we are here and that we have tanks so they won't try to push through our roadblock," he added. "We'd only fight if they attacked us."

President Oueddei's spokesman, Ahamad Alkhali Macka, said in an interview that the Chad government was pressured by France, the United States and African governments to ask the Libyans to leave. Oueddei had asked them to help in the struggle against Habre in 1980.

When the Libyans were here there was peace, Macka said. "Now this OAU force has replaced the Libyans and is supposed to be a force to maintain peace but says it is a neutral force," added Macka. If Habre has broken the peace then the OAU force should have attacked him, Macka continued. "Have they come to Chad to walk around like tourists?" he asked.

The force's commander, Maj. Gen. Geoffrey O. Ejiga of Nigeria, said his instructions are to maintain peace without taking the side of any of the country's 11 factions. "We are here to help create the conditions for all sides to come to the negotiating table," he said.

"The problem of Chad," Ejiga said, "cannot have a definite military conclusion. The solution here will have to be political."

Ejiga's present force of 3,800 Nigerians, Zairians and Senegalese was supposed to grow to 6,000 with additions from Togo, Benin and Guinea. The latter three countries have yet to send troops because they lack the money. A full, 6,000-member force could cost more than "$80 million annually," Ejiga said. So far, France has contributed an unspecified amount to the Senegalese contingent while the Reagan administration donated $12 million to the OAU for the force and nonlethal military equipment to the Nigerian and Zairian units.

Last week, Kenyan Foreign Minister Robert Ouko announced that the OAU will hold an emergency meeting in two weeks on the question of financing the peace-keeping force. There also will be a debate on the differences between the OAU's role and the Chadian government's desire that it fight Habre, Ouko said.

Several well-informed sources said that the OAU will ask the United Nations for funds. "It's unclear if the U.N. would vote to fund a force that is not directly under its control," said one source.

Indications are that the need for a peace-keeping force in Chad will exist for years because none of the factions is strong enough to overcome the others.

Oueddei has stated several times that he is willing to negotiate with the faction that Habre leads, but will never negotiate with Habre. Macka, Oueddei's spokesman, said there have been three battles for Ndjamena since 1979 because Habre broke three negotiated agreements.

Several neutral observers agree that Habre has a reputation for being untrustworthy. "Habre is the most ruthlessly ambitious and brilliant" of the faction leaders, said one neutral source. "He has broken every agreement he has made," the source added. "No one will or could trust him and none of the Chadian factions, either separately or jointly, wants to negotiate with Habre," he said.