About 5,000 Libyan-backed rebels launched a major assault on the key Chadian town of Faya Largeau today only three hours after the first small contingent of French soldiers arrived inChad's capital of Ndjamena to bolster the beleaguered forces of President Hissene Habre.

A spokesman here for rebels, led by Chad's former president, Goukouni Oueddei, claimed that Faya Largeau was now under "100 percent control" of Goukouni's Libyan-supported "Chad Government of National Unity." U.S. diplomats here monitoring the situation said it was their understanding that "Faya has fallen."

But Chadian Information Minister Soumaila Mahamat told reporters in Ndjamena that Habre's forces had "decisively defeated" and driven back the rebels. Western military sources in the Chadian capital said the situation was regarded as "extremely critical." They added that all radio contact with the northern oasis had been lost at midday, and they could not confirm its status.

At a news conference this morning, Soumaila said the attack was launched by two armored columns of 2,000 Libyan regulars and about 3,000 rebel soldiers supported by tanks, heavy artillery and aircraft, wire services reported. He said the 2,500-man government garrison had forced a rebel retreat.

Faya Largeau, a desert town of 7,000 people, was overrun by Goukouni's forces in June and retaken by government troops July 31. Since then, it has been the target of repeated air attacks by what Habre, France and the United States have charged are Libyan fighters.

Libya has denied involvement in the conflict. In Paris, Goukouni's spokesman, Abderrahman Moussa, told Agence France-Presse that the situation "has now taken on such proportions that . . . Goukouni will not be able to hold out alone for much longer." He added that Goukouni's forces would be forced to call on "friends," including Libya, for aid.

Moussa said Goukouni's forces "much regretted" France's decision to send 180 soldiers as "instructors" to Habre's government. He said the move was an indication that Paris had yielded to "pressure from moderate African states" and "American blackmail," AFP reported.

The French movement of troops into Chad, code named Operation Stingray, began last night when an advance guard of 30 paratroopers was ferried across the Chari River from neighboring Cameroon.

The remainder of two companies, dispatched from bases in the Central African Republic, to the south of Chad, were scheduled to arrive in Ndjamena by Thursday.

The government initially announced that 180 men would be deployed as advisers in Chad, but the newspaper Le Monde reported today that the total contingent was expected to reach 314.

U.S. diplomats here, who have strongly reflected Washington's hope that France would answer Habre's urgent request to send fighter aircraft, today expressed deep concern at the rapidly unfolding events in the former French colony. Asked if the United States still hoped France would send jet fighters to counter the Libyans, one U.S. diplomat responded, "That would be like locking the barn door after the horse was stolen."

Following yesterday's announcement here that paratroopers would be sent to Chad in an advisory capacity, French politicians and commentators today reacted with apprehension and worry that France might become mired again in the ceaseless internal wars of its former colony.

Even though Mitterrand insists that French troops will not undertake any combat role, Defense Minister Charles Hernu fueled speculation yesterday that France might soon escalate military action to counter Libyan intervention.

"France did not take the initiative to internationalize the conflict; the Libyans did," Hernu said. "Everything that Libyans do, we will also do, except the bombing of civilian populations."

Hernu's vow to meet the Libyan challenge generated serious qualms among French officials that growing pressures from the Reagan administration and conservative African leaders might erode Mitterrand's strong resistance to becoming embroiled in postcolonial disputes.

Some Socialist Party members were said to have labeled the decision to send troops back to Chad after an absence of more than three years "a setback" to government policy.

The Communist Party, the Socialists' junior partner in the ruling coalition, opposed the move and warned that "France must not stick its feet in the Chadian quagmire."

The French troops were said to be equipped with large amounts of radar and communications gear, possibly to enable them to receive and relay signals from U.S. AWACS reconnaissance planes stationed in Khartoum, Sudan, to Chadian forces.

The AWACS, however, are not expected to be deployed unless the French send into action some of the 20 Jaguar or Mirage III jet fighters poised at French air bases in neighboring African countries.

The Mitterrand government has rebuffed pleas to dispatch the aircraft, maintaining that bombing and strafing runs conducted against rebel positions would violate a 1976 agreement that forbids any combat role by French forces on Chad's territory.

But Mitterrand's reluctance to become deeply enmeshed in the desert civil war goes much deeper than the letter and spirit of that accord.

He seems determined to put an end to more than two decades of French intervention in the incessant civil wars of the landlocked state.

As the diplomatic and military pressures have intensified, Mitterrand has betrayed no signs of being persuaded by the Reagan administration that Libya's support for the rebels represents a push for a pro-Soviet axis across the Sahara. Until the civil war erupted again in June, France was ardently seeking to improve relations with Libya's mercurial leader, Muammar Qaddafi.

Mitterrand and his top aides also feel no great admiration for Habre and do not perceive him as a pro-Western leader beset by hostile Soviet surrogates.

During Habre's days as a rebel leader, when France was seeking to prevent him from toppling the Goukouni regime that was deemed at the time to be the sole legitimate government, his forces kidnaped a French woman, Francoise Claustre.

Her detention enraged French public opinion and aroused so much resentment toward Habre that he has found little sympathy here for his troubles with Qaddafi.