Four Jaguar combat jets made screeching passes low over this capital today before landing at the military airport. The planes are the latest addition to France's military buildup in defense of the government of Chadian President Hissene Habre against rebel forces backed by Libya.

The ground-support Jaguars were joined by an air tanker and four F1 Mirages, France's most advanced aircraft, which would provide air cover for the Jaguars. Arrival of the planes brought a significant change in the nature of the civil war here.

Until now, Habre's Army, which is dug in alongside French paratroops and Legionnaires at northern battlefront positions, had been virtually without air support to counter the reported extensive bombing and strafing by Libyan planes in support of the rebels.

Habre has asked repeatedly for French aircraft, and U.S. officials have pressed publicly for the former colonial power to provide air cover here. The United States has made available radar-control planes to coordinate use of French warplanes.

Chad has no air force. Zaire did send three older-model Mirages as part of its military aid to the government.

Yves l'Ancien, a member of the French National Assembly's defense committee, said in Paris last week that there are 92 aircraft in the Libyan-occupied "combat zone" of northern Chad.

A French military spokesman said only that there are "four to 20 Jaguars in the Chadian region" when asked about reports that an additional 12 of the jets are in the Central African Republic south of Chad.

The Jaguars "will intervene to defend French positions" in any future fighting, the spokesman added. As French troop positions are practically indistinguishable from those of Habre's forces, it was thought here that Habre's soldiers also would benefit from the air cover.

Asked if the French Air Force would provide cover for Habre's troops if they attacked the rebel-held and heavily fortified town of Faya Largeau, the French spokesman answered, "I have nothing to say about any possible government counterattack missions."

The arrival of the jets had been expected here for several days in what appears to be a three-week tit-for-tat military buildup of French and Libyan armaments for the sides each is backing in this phase of Chad's 18-year-old civil war.

There has been an undeclared cease-fire since Libya stopped bombing Habre's troops at Oum Chalouba Aug. 12. Since then, a western diplomatic source said, Libya has poured armaments "by air and land" into Faya Largeau. France has been doing the same for Habre's government troops, bringing in more than 1,000 soldiers this month. The fighting began with an attack by the rebels on Faya Largeau in early June.

The Jaguars here flew in from a French base in neighboring Cameroon. The Jaguar is a joint product of the French and British, listed as having a range of about 800 miles when armed.

Rebel-held Faya Largeau is 625 miles northeast of Ndjamena. Chadian, French and Zairian forces, the latter also aiding Habre's army, are positioned up to 200 miles south of Faya Largeau at Salal, on the western route to the capital, and around Arada, Biltine and Abeche, on the eastern approach to the capital.

The U.S.-made French KC135 tanker here "is capable of in-flight refueling of the Jaguars," the spokesman said. Travelers saw two KC135s at the Bangui airport in the Central African Republic last week.

The Soviet-made jets that were used by Libya are reported to be based near the Libyan-Chadian border and to be out of range of Ndjamena. They have no in-flight refueling capacity, according to intelligence sources.

Thirty-four French journalists cabled a protest to Defense Minister Charles Hernu yesterday accusing the French military here of "blocking" coverage of the war and of not providing the repeatedly promised air transport to the front while forbidding privately rented planes to land at the government-held town of Abeche.

The spokesman here said Hernu had responded by saying that the reporters here, for the moment, would not be transported to Abeche, but they could go by private plane if the Chadian government gave them clearance. There are about 130 foreign reporters here.