With outsiders charging that Libyan troops have not left Chad - as promised - French President Francois Mitterrand flew to Crete Thursday for a rendezvous with Libyan leader Muammar Qaddafi.

French officials here insisted that the meeting was not meant to dramatize any new confrontation over Chad.

The officials repeated the French assertion that all Libyan troops have left the African country, and that the meeting was only possible because of the withdrawal agreement between the two countries.

''The meeting became possible,'' insisted government spokesman Roland Dumas, ''when the agreement on evacuating Chad took effect.

''President Papandreou wished to meet President Mitterrand to talk about bilateral problems, and he suggested that it would be profitable to combine this visit with the visit of the Libyan head of state. President Mitterrand agreed.''

But the visit comes just as doubts are rising over the Chad withdrawal agreement. Chadian President Hissein Habre insists that most of Libya's 5,500 troops remain in occupation of the north of his country.

American State Department spokesman John Hughes has said that American intelligence also showed a continued Libyan presence.

The French respond that their observers have confirmed the Libyan withdrawal. They have not, however, made public any specific proof.

While one state television crew filmed a departing column of Libyan vehicles, the reporter admitted he was not allowed to go to Faya-Largeau or Fada, the towns where most of the Libyan troops are supposed to have been stationed. French television has shown extensive footage of French troops leaving the south of the country.

Why has the affair turned into a polemic?

According to the French, it is the result of a continuing difference of opinion between Washington and Paris on how to deal with the Libyans.

The French criticize the Reagan administration for trying to isolate Colonel Qaddafi diplomatically. The French insist that negotiations are possible with Qaddafi, provided words are backed up with force.

French troops were sent to Chad in August 1983 to stop the invasion of Libyan-backed rebels led by Goukhouni Woddei. The Libyans refused to confront the French troops. The ensuing standoff continued until this September, when Libya signaled it was tired of having its troops tied down in the desert.

The troop withdrawal agreement followed, along with two French messages. The first was the threat that French troops would return if Libyan troops once more attacked Chad. The second was the suggestion that President Mitterrand was ready to meet with Qaddafi.