France sent a unit of military instructors to Chad yesterday, signaling an escalation in its involvement in the African country's civil war but stopping short of the dispatch of combat aircraft being urgently requested by Chadian President Hissene Habre.

In announcing the decision to send about 180 military advisers to Chad, where Libyan aircraft reportedly renewed heavy bombing raids against government forces Monday, Defense Minister Charles Hernu said yesterday in Paris, "We are there as instructors, as teachers of the Chadian Army."

While Hernu's announcement marked a reversal of France's reluctance to increase the scope of its involvement in Chad, it appeared unlikely to meet Reagan administration desires that Paris agree to Habre's request for air support. Administration officials said yesterday that the two AWACS surveillance planes sent to Sudan to monitor Libyan bombing raids would not be used unless France dispatches fighter aircraft to counter Libya's air and logistical support of the rebels. Details on Page A19.

An official at the French Embassy here said the instructors "are not going to participate in any fighting." The official said the advisers already are en route from bases in the Central African Republic, a neighboring former French colony.

The French Embassy official said there was "no indication" that France would send fighter aircraft to Chad. The official also said the instructors would not go to Faya Largeau, a desert town 500 miles from the capital where the heaviest fighting is occurring between government troops and the Libyan-backed rebel forces led by Goukouni Oueddei. Libya is believed to be providing ground support to the rebels in addition to flying almost daily bombing raids over Faya Largeau.

Hernu said the dispatch of instructors to Chad is "in accordance with the first article of the 1976 agreement," which requires France to supply military support to Habre's government. While Hernu made statements Sunday widely interpreted to mean that France would not send aircraft or troops to Chad, Foreign Minister Claude Cheysson declared on Monday that France "cannot be indifferent" to Libya's role in Chad, warning that "there will be consequences" of further Libyan support of Goukouni's forces. Goukouni is the former president of Chad ousted by Habre last December.

French President Francois Mitterrand, a Socialist elected in May 1981 on a platform opposing a long history of French intervention in Africa, has been reluctant to send French military personnel to Chad. But with continued Libyan bombing raids on Faya Largeau being viewed as a test of France's willingness to support its African friends--and consistent pressure from the United States and African nations to intervene--Mitterrand has stepped up involvement in the former French colony.

Sending military trainers to Ndjamena is viewed as an attempt to demonstrate further French resolve without committing its troops or planes to combat duty. The aim is to ward off continued Libyan involvement in Chad. While some observers believe France is likely to gradually escalate its involvement in Chad if Libya maintains its current level of activity, others believe Mitterrand will do so only in response to a sharp increase in Libyan activity.

Meanwhile, Chadian government officials said Libyan fighter aircraft continued bombing raids over Faya Largeau yesterday, dropping phosphorus and fragmentation bombs on the besieged town. According to news agencies, Chadian Information Minister Soumaila Mahamat said two Soviet-built combat aircraft, piloted by Libyans, dropped 500-pound fragmentation bombs and napalm on residential areas.

There were no casualty reports from the latest bombing raids, but Mahamat reportedly said "several hundred" of Faya Largeau's civilian population had been killed in bombing raids since the town was recaptured by government troops at the end of July.