Prime Minister Laurent Fabius committed France's Socialist government today to take "all necessary measures" to ensure the withdrawal of Libyan troops from Chad.

The prime minister's pledge, which was made before a noisy session of the National Assembly here, coincided with the publication of controversial intelligence information suggesting that the Libyan presence in the former French colony is much greater than previously acknowledged here.

The new figures have come as a further embarrassment to President Francois Mitterrand at a time when he is already seeking to defuse a row triggered by his meeting last week with Libyan leader Muammar Qaddafi.

The government of Chad, meanwhile, said that an army patrol in northern Chad had been fired upon from a Libyan helicopter yesterday in the first such incident since the completion of the French withdrawal Nov. 10. The incident is likely to be used by the Chadian authorities as evidence that Libyan troops still in the country have not limited themselves to a purely defensive role, as claimed by the French government.

Quoting a memorandum prepared for the president by an official French intelligence gathering organization, the independent leftist daily Liberation reported today that 3,000 Libyan troops were still in Chad. The report said that the Libyan units, which included some "fresh troops," were equipped with heavy armor, including 57 tanks, seven helicopters, and ground-to-air missile batteries.

French government spokesmen sought to depict the estimates for Libyan troop strength, which coincide with official American reports, as ridiculously inflated and contradicted by other, more reliable information available to the Ministry of Defense. Officials, however, did not deny the authenticity of the document attributed by Liberation to the General Secretariat for National Defense, a government body that collates and distributes military intelligence.

At a press conference Friday after returning to Paris from a meeting with Qaddafi Thursday on the Greek island of Crete, Mitterrand said there were about 1,000 Libyan soldiers still in Chad in violation of a joint troop withdrawal agreement between Libya and France. He insisted, however, that the Libyans did not have any heavy armor such as tanks. Earlier, official French spokesmen had maintained that all Libyan troops had left Chad.

Addressing the National Assembly today, Fabius said that the French government had been aware at the time of the Crete meeting that the Libyan withdrawal was not complete. He added that Mitterrand had nonetheless decided to meet with Qaddafi in an attempt to avoid unnecessary casualties to French soldiers in the event of a confrontation with Libya.

"We are not warmongers," Fabius repeated three times amid cheers from his own supporters and jeers from the right-wing opposition, which has stepped up its attacks on the government's handling of the affair over the past few days.

He added: "France respects and will respect its commitments. It is our tradition to do so. The Libyan side must do the same. The necessary measures for any eventuality will be taken to see that this respect for the troop withdrawal agreement will be total. We are saying this as the leaders of France."

The prime minister's statement was deliberately ambiguous about the methods Paris intends to use to see that Libya complies with the troop withdrawal agreement reached in September. It is nonetheless important as it appears to represent a public undertaking by the French government to ensuring that Qaddafi adheres to his word.

The preferred solution to the crisis from the French point of view would clearly be a peaceful Libyan withdrawal in the next few days. At a press conference earlier this week, the Libyan leader said he was totally committed to the troop withdrawal agreement with France and blamed "logistic difficulties" for the failure to implement it on time.

Political analysts here are extremely skeptical about such vague assurances from such an unreliable source. In the absence of any concrete evidence that Libyan troops are indeed being pulled out, speculation here has centered on the limited range of military options open to Mitterrand, who is faced with the task of reestablishing his own and France's political credibility in black Africa.

Politically cautious, Mitterrand is likely to do his utmost to avoid a direct clash between French and Libyan troops in the deserts of northern Chad. He is, however, under heavy pressure from the Chadian government to do more than simply restore the previous status quo which allowed the Libyans and their allies to control the north of the country.

Chadian leaders have urged the French to adopt a much more dynamic and agressive role in the event of their return. One possibility openly mentioned by Chadian officials has been the provision of French air cover to allow the small but relatively well-equipped Chadian Army to recapture the northern oasis towns from the Libyan-backed rebels.

France resumed reconnaissance flights by Jaguar aircraft over Chad earlier this week from bases in the neighboring Central African Republic.