Paris — The Chad crisis won't go away. As reports mount that the French have underestimated the size and strength of Libyan forces in the north of Chad, President Francois Mitterrand faces a difficult choice:
* He can remain silent and hope Libyan leader Muammar Qaddafi eventually keeps his word to withdraw. But this risks eroding his credibility both at home and abroad with Washington and France's traditional allies in Africa.
* Or the French President can redeploy his troops, and thus risk a military escalation that could turn into a quagmire.
''The question is whether France will be at peace or at war,'' Prime Minister Laurent Fabius warned solemnly in his presentation of the government's position before the National Assembly.
So far, the government is giving peace a chance. Neither Mr. Mitterrand nor Mr. Fabius has publicly set a deadline or threatened specific reprisals. There has been only a minimal amount of French saber-rattling, putting 250 troops stationed in the neighboring Central African Republic on alert and sending two Jaguar aircraft on a reconnaisance mission over Chad.
The hope is that the promise of improved ties with Paris will eventually induce Col. Qaddafi to respect his side of the withdrawal pact.
But the strategy does not seem to be working. Reports here say that Libya has reinforced its presence and returned to the offensive. The usually pro-government newspaper Liberation reported that French intelligence now believes Libyan forces total some 3,000 men with heavy equipment, not the 1,000 men with light support that Mitterrand has claimed. And the Chadian government says that Libyan planes have attacked their forces.
Each day that passes without a French reaction results in increased criticism of Mitterrand. The domestic damage has been exacerbated because it comes at a time when the President's standing has fallen to an all-time low in the polls, and because foreign policy was the one area in which Mitterrand was universally admired.
That admiration has waned. The opposition has been attacking the President as weak and vacillating, even foolish. It also scored points by criticizing his refusal to give any indication of what agreement, if any, he and Col. Qaddafi reached at their Nov. 15 meeting in Crete.
''We're in a complete fog,'' commented Pierre Messmer, a former Gaullist prime minister.
Complaints have come from presidential friends. Serge July, the influential Liberation editor, compared Mitterrand's mistake to ''descending a steep stairway and missing a step that throws off one's equilibrium.''
Mitterrand's task abroad is just as great. He must first straighten out the spat over Chadian policy with the Americans. It was the US state department that first disclosed that Libyan troops had not withdrawn, and last week's visit to Washington by French Foreign Minister Claude Cheysson disclosed deep policy differences on how to deal with Libya.
The Americans believe Qaddafi is an international terrorist who cannot be dealt with. The French believe it is ''a political mistake'' to try to isolate the Libyan leader, because it only encourages him to destabilize other regimes. Better, Cheysson says, to contain his adventuring by negotiating with him.
The French have a similar disagreement with their African allies. France provides ''a defense umbrella'' for the former French colonies, and it was largely pressure from them that originally persuaded Mitterrand to intervene in Chad. The Africans remain wary of Libyan designs and expect France to help.
Chad Foreign Minister Gouara Lassou says French soldiers must not simply return - ''They must fight.''
The annual Franco-African summit is scheduled for Dec. 11-12 in Burundi, and Mitterrand must want to prove his fidelity to his African friends by that date.''