France stepped up a new war of nerves with Libya's Col. Muammar Qaddafi today by placing 250 paratroops on a state of readiness to move back into Chad unless Libyan forces are withdrawn from the north of the country.

The French move appeared designed to increase the pressure on the Libyan leader while allowing him more time to withdraw all his troops from the former French colony. According to French estimates, Libya still has around 1,000 soldiers in Chad after failing to respect an agreement to complete withdrawal of its troops by Nov. 10.

Speaking to reporters on the Mediterranean island of Malta today, Qaddafi said he was "totally committed" to the troop withdrawal agreement negotiated with France in September. He said that, in the future, Libya would only move into Chad if another country went in first.

While insisting that French troops will be sent back to Chad if the Libyans remain, French officials refused to give the mercurial Libyan leader a specific deadline to keep his word. But President Francois Mitterrand's Socialist government is now coming under political pressure, from both at home and abroad, to show that it has not been duped by Libya into withdrawing unilaterally from Chad.

At home, the affair has become a welcome political gift for the right-wing opposition, which has accused Mitterrand of jeopardizing France's credibility. One leading opposition politician, Francois Leotard, said today that France had been made to look like a "paper tiger," while the newly elected secretary general of the Gaullist party, Jacques Toubon, accused the Socialists of being in a hurry to get out of Chad before legislative elections scheduled for 1986.

Mitterrand's decision to meet with Qaddafi last week on the Greek island of Crete has strained generally excellent relations between Paris and Washington and caused concern in moderate countries in Africa such as Egypt.

French Defense Minister Charles Hernu, who flew to the Chadian capital of Ndjamena yesterday, held 3 1/2 hours of talks with Chadian President Hissene Habre today on the possible return of French troops. After he flew back to Paris, French television quoted him as saying there was no question of French troops returning to Chad for the moment but that reconnaissance flights by French Jaguar aircraft would continue.

The French troops placed on alert today are stationed in the Central African Republic, which borders on Chad. According to a report by the French news agency Agence France-Presse from Bangui, they were told that they could be sent to Chad.

French commentators noted that Qaddafi did not deny that Libyan troops were still in northern Chad at his press conference in Malta today. Tripoli radio erroneously had quoted Mitterrand earlier as denying "the allegations of the imperialist American media concerning the maintenance of the Libyan military presence in Chad."

Qaddafi described President Reagan as "madman" who would lead the United States and the world to disaster. He also called Egyptian President Hosni Mubarak "an agent of the American counterintelligence agency."

Mubarak has accused Libya of plotting to kill a number of world leaders, including Mitterrand and West German Chancellor Helmut Kohl, along with former Libyan prime minister Abdul Hamid Bakoush. A plot to assassinate Bakoush was foiled by Egyptian security services last week.

Both Mitterrand and Kohl brushed aside the assassination scare at a joint press conference in Paris after an informal summit meeting. Mitterrand said he would wait and see "what happened" while Kohl said he would wait "calmly."