Beleaguered Chadian President Goukouni Oueddei today formed a new government as he faced a deadline next week to accept an inter-African ultimatum to negotiate with rebel forces or to lose African peace-keeping troops.

The new Cabinet in the former French colony in central Africa included almost all new ministers. But without meaningful outside support from Algeria and Libya, which at one time backed the government, French observers doubted that Goukouni's inclusion of rival war lords' representatives on the Cabinet could stop the rebels.

French officials suggested privately that the rebel troops, led by former defense minister Hissene Habre, could invade the capital, Ndjamena, almost at will. The force is estimated to number perhaps as many as 8,000 troops and has advanced to less than 100 miles of the capital.

Goukouni's latest attempt to keep his government in office began when he recently made a trip to Libya and returned empty-handed.

After first welcoming Libyan troops in 1980 to help prevent a Habre victory, Goukouni last year stunned the world by persuading Col. Muammar Qaddafi to remove them.

Goukouni has refused to negotiate a compromise with Habre, a demand made by the OAU as its price for maintaining Nigerian, Senegalese and Zairian peace-keeping troops in the country. Gradually, Goukouni has been abandoned by his one-time domestic allies who controlled separate private armies.

Last month the OAU warned Goukouni that the peace-keeping troops would be withdrawn unless he agreed to a cease-fire with the Habre forces by June 10.

The only encouraging development for Goukouni was Nigeria's decision to renew the mandate of its troops until the end of July. The other two troop-contributing countries have promised to maintain their forces only until June 30.

So far, Habre's one-time backers during the Libyan occupation period--especially the United States, Egypt and neighboring Sudan--have sworn they have stopped supplying his troops.

However, even in French offical circles, some analysts are not convinced that the United States has cut all ties with Habre.