France announced today that it has appointed a veteran paratroop commander to oversee French troops helping Chad counter a Libyan-backed rebel movement.

Marine Gen. Jean Poli, 54, a veteran of France's war against Algerian nationalists in the 1950s and 1960s, was named to replace paratroop Col. Bernard Messana as overall commander of French forces here.

Poli was recently commander of the several thousand troops France maintains in the small East African nation of Djibouti and has served as a military adviser to Zairian President Mobutu Sese Seko, who has sent 2,500 troops to help the Chadian government of Hissene Habre in its battle against rebels led by former president Goukouni Oueddei.

Meanwhile, informed French sources said Paris is sending fighter aircraft here, in what is turning into the biggest French military operation on this continent since the war against Algerian nationalists two decades ago.

While the sources said the airplanes included an unknown number of attack Jaguars and France's top-of-the-line Mirage combat jets, a high-ranking French military source refused to confirm or deny that the planes will be coming, their numbers or reports that they will be landing tonight or Saturday morning.

France did not want to commit the planes to the defense of Habre's government, the informed sources said, until they had installed a "sophisticated antiaircraft" missile system around Ndjamena.

"The system is now in place," one source said.

Paul Webster of the Manchester Guardian reported from Paris that France is ready to fly both supersonic Jaguar fighter-bombers and Mirage fighters to Ndjamena this weekend.

Four Jaguars are based at Bouar in the adjoining Central African Republic about 540 miles from the front line. A move to Ndjamena would bring them within 240 miles of positions held by rebel and Libyan forces in the north of Chad. Another four Jaguars are based at Niamey in Niger, about 900 miles from the battle lines but much nearer Libyan Air Force bases in the disputed Aouzou Strip between Chad and Libya.

Six Mirages are based at Cap Vert in Senegal and could be flown nearer to Chad to cover the continuing airlift of men and materiel against possible air raids.

The transfer of fighter-bombers and fighters so close to the front would tilt the war clearly in favor of Chadian government troops as the front lines are out of range of Libyan Air Force jets that played a key role in the fall of Faya Largeau, the northern oasis, to rebel troops 10 days ago, Webster reported. Defense sources here indicated that by moving up planes to Ndjamena any attempt to make the Faya Largeau airport serviceable for the Libyan Air Force could be checked easily.

The French commitment of fighter planes changes the power balance of this war. Until now, Libya's Soviet-made bombers and fighter planes had unchallenged control of Chad's airspace. Libya's last bombing and strafing attacks were conducted against Habre's forces in the northern town of Oum Chalouba last Friday.

There has been an undeclared cease-fire, the first since the fighting began in late June, since last Friday. More than 1,000 French troops and 2,500 Zairian troops are allied with Habre's force of fewer than 4,000, on one side, while there are about 2,500 Libyan Army regulars, an unknown number of Libya's multinational Islamic Legion and an unknown number of Chadian rebels nominally led by Goukouni, on the other.

Chadian Information Minister Mahamat Soumaila said today that Libyan leader Muammar Qaddafi "wants to build an empire" of the countries in northern Africa, including Chad. "It is a bizarre concept in the 20th century," Soumaila said.

Qaddafi "has tried to overthrow the governments of Niger, Sudan and Mali," Soumaila said. Each of the three governments has, in recent years, separately accused Qaddafi of financing clandestine opposition movements, recruiting or forcing their nationals into Libya's Islamic Legion or attempting to stir up domestic ethnic antagonisms through radio broadcasts.

"Qaddafi confuses Arabism with Islamism, treating them as one and the same," continued Soumaila. "It is only force that will stop him," he concluded.