Increasingly concerned about clashes between rival armies in Chad and Libya's growing occupation role there, France has stepped up pressure on friendly African governments to dispatch a joint peace force to the ravaged central African country.

The new sense of urgency in Paris grows from fears that strife within the National Union Transition Government, headed by President Goukouni Oueddei, and the Sudanese and Egyptian-backed revolt of former defense minister Hissene Habre will give Libyan leader Muammar Qaddafi reason to prolong and strengthen his Army's hold on Chad.

It also comes against a background of increased Western nervousness -- particularly in the Reagan administration -- over Qaddafi's influence in northeastern Africa following the assassination of president Anwar Sadat of Egypt earlier this month.

Although nothing has surfaced to suggest Libyan involvement in the murder, Secretary of State Alexander M. Haig Jr. has reiterated U.S. determination to resist Libyan-sponsored subversion in Egypt and neighboring Sudan since Sadat's death.

Sudanese President Jaafar Nimeri has cited Libyan bombing of Sudanese border villages, used as havens by Habre's irregulars, as proof of Libyan aggression and an indication of Qaddafi's intention to invade Sudan. Reports from Khartoum, the Sudanese capital, suggest Western diplomats see less danger of Libyan attack. But Haig, in his effort following Sadat's assassination to show that Washington stands by its friends, said the United States will speed deliveries of weapons to Nimeri's Army.

French concerns center on Chad because of a colonial and post-colonial role there that lasted militarily until then-president Valery Giscard d'Estaing pulled French troops out of the chaotic, multi-sided civil war. The conflict was finally settled only when Libya intervened on behalf of Goukouni nearly a year ago with an expeditionary force now estimated at more than 8,000.

The new government of President Francois Mitterrand is determined not to renew France's military role in Chad, judged by his Socialist Party as neocolonialist and unwise. At the same time, Goukouni openly has sought French help in rebuilding a national army and restoring Ndjamena, the heavily damaged capital.

In addition, Paris is eager to regain the confidence of its African allies. These leaders were shaken by Giscard's inaction as Libyan troops rolled into Chad to aid Goukouni in what was seen as a major blow to French and Western influence in Africa and a boost for Qaddafi's dream of extending his radical brand of Islam across the continent.

During a visit here by Goukouni last month, Mitterrand's government pledged to help rebuild Ndjamena even while Libyan troops remain, a concession from the earlier French position.

Simultaneously, French envoys have been pressing the Organization of African Unity to act. The OAU resolved after a summit meeting this spring to send an inter-African peace force to Chad by July to replace the Libyan forces. Paris offered to help finance and transport the force, but African leaders seem relucant to commit troops to the Chadian quagmire despite the commitment.

Now sharp clashes have broken out near Mongo in central Chad between forces headed by Interior Minister Mohammed Abba Said and rival bands led by Foreign Minister Ahmed Acyl. Libyan troops are reported here to have intervened on Acyl's side, underscoring the occupation army's key role in Chadian affairs.

At the same time, French officials have reported indications that Libyan troops in eastern Chad, near the area where Habre's rebels operate from their Sudanese sanctuaries, have begun assuming civilian administrative roles as well as military duties. This has reinforced fears here that unless the OAU acts quickly, Qaddafi's troops will settle in to stay, perhaps accomplishing the "merger" he and Goukouni have spoken of on several occasions but never consummated.

The Libyan leader dramatized these concerns in an interview broadcast last night on Italian state television in which he said Nimeri's support for Habre makes it impossible for Libyan troops to leave for home this year as Qaddafi had promised they would -- and that indeed they will be strengthened if Nimeri and President Hosni Mubarak of Egypt increase their support of Habre.

"Above all, after the clear and continuing military threat of Nimeri against Chad," Qaddafi said, "a withdrawal of our forces in the immediate future has become unfortunately -- I say unfortunately -- impossible, despite the fact that we had hoped to pull out by the end of the year."

Against this background, Mitterrand dispatched a cable from the Cancun summit conference to President Daniel arap Moi of Kenya, the current OAU president, urging that the pan-African peace force be formed and sent to Chad "without delay." French Foreign Minister Claude Cheysson followed up, declaring a vanguard of the OAU force should be in place within a week.

Although Foreign Ministry officials here were unable to explain the sudden urgency, they said Mitterrand and Cheysson had become alarmed by the fighting within factions in the government and the reports of Libyan administration in eastern Chad. Press reports said African leaders remain reluctant to commit troops to the OAU force, with only Senegal, Ivory Coast and Nigeria expressing tentative accord despite repeated French prods