The government of Chadian President Hissene Habre said today that the strategic northern oasis of Faya Largeau succumbed to what it described as a massive assault launched early yesterday by 5,000 rebels and Libyan soldiers.

During the six-hour attack, marking the first time that Libyan troops have participated openly in Chad's 19-year-old civil war, the rebel forces were reported by western military sources in Ndjamena, the capital, to have deployed waves of Soviet-built jet bombers, tanks and artillery to drive the bulk of Habre's Army from Faya Largeau.

The rebels had announced their recapture of the city yesterday. It last changed hands July 30.

Chad's information minister, Mahamat Soumaila, told reporters in Ndjamena that the government's forces had retreated to positions five miles west and 11 miles east of the outpost "to spare the 7,000 civilian inhabitants further casualties and suffering."

The heat-seeking Redeye antiaircraft missiles recently supplied by the United States to thwart Libyan bombing runs were withdrawn before the climactic battle and did not fall into the hands of rebel forces, according to western military sources in Chad.

In Washington, it was reported that the missiles had failed to work, apparently because of problems caused by the extreme heat in Chad.

Asked why the Redeye missiles were pulled out of Faya Largeau before the attack, Soumaila said: "It would be wrong to claim that the Redeyes were not functioning well. But the Libyan air attacks were so overwhelming that they could not be effectively deployed."

Officials at Chadian Embassy in Paris said that the government's forces were defenseless against Libyan bombing raids carried out by what one described as "more than 50 jet fighters in the area."

The rebel victory, occurring only hours after the first contingent of French paratroopers arrived in Ndjamena on what was described officially as a training mission, was expected to intensify pressures on French President Francois Mitterrand to dispatch more soldiers and combat aircraft to resist the Libyan-backed advance.

President Mobutu Sese Seko of Zaire, who along with other pro-Western leaders of black Africa has pleaded for a more assertive French role in the conflict, stopped in Paris today on his return from Washington to pay a 40-minute "courtesy visit" on Mitterrand.

"Chad must be able to count on its friends to reestablish its territorial integrity," Mobutu told reporters later at the Elysee Palace. "The aggressor is Libya and the victim is Chad, which must not be left alone in this affair." But Mobutu dodged a question about more direct French involvement in the civil war and said only that Mitterrand could decide whether France should escalate its military measures in the region.

Defense Ministry officials tonight denied reports that France would send as many as 500 troops to handle training and communications tasks for Habre's Army. They said that as of late Thursday, 180 men had landed in Ndjamena and that ultimate deployment was expected to be "about 250 or 300 French troops."

"We are living up to our commitments," said an adviser to Defense Minister Charles Hernu. But he refused any comment on whether France might send the fighter aircraft and combat troops that Habre has desperately requested to turn the tide of battle.

France claims it will abide by a 1976 agreement with Chad that forbids any combat role for French forces on Chad's soil. French spokesmen point to $40 million in equipment supplied to Habre as proof that Mitterrand's government is meeting its responsibilities.

Defense Ministry officials said Mitterrand finds the idea of military intervention repugnant, but they admitted that "a vast debate" is now taking place within the government to decide what to do if the Libyan-backed rebels attempt to push beyond Faya Largeau toward the capital 500 miles south.

Some French officials were surprised that Libyan Col. Muammar Qaddafi committed so many troops and such firepower in the attack on Faya Largeau. According to accounts available here, as many as 2,000 Libyan soldiers--carrying out a pincer assault with heavy artillery and tanks--clearly provided the brunt of power in the rebel thrust while Libyan pilots in Soviet-built MiG fighters conducted unmolested bombing runs.

Mitterrand's advisers reportedly had thought that the imminent arrival of French paratroopers in Chad, albeit for training purposes, would dissuade Qaddafi from sending troops on the full-fledged assault.

The French president reportedly criticized his key advisers on Chad, Africa specialist Guy Penne and Mitterrand's son, Jean Christophe, for not anticipating the gravity of the crisis.

A consensus of French experts within the government, however, is said to believe that Qaddafi is not likely to court a confrontation with French forces by marching on Ndjamena, despite the evident desire of the rebel leader, ex-president Goukouni Oueddei, and his forces to regain power.

France maintains 6,500 well-trained and well-equipped troops in neighboring African countries. Any Libyan-led assault on the training contingent now in Chad almost certainly would unleash a tough response by the French forces in the region.

Qaddafi's goals remain unclear, but many analysts believe that his primary aim is to secure full control of northern Chad, which is dominated by nomadic Moslems.

Since 1973, Libya has occupied a band 60 miles wide of northern Chad known as the Aouzou Strip. The area is rumored to be rich in uranium and manganese, but no recent tests have verified this claim.

The southern part of Chad, with the more populous capital of Ndjamena largely inhabited by Christians and animists, may prove too formidable to incorporate because of religious and ethnic reasons.