Rebel troops in Chad, with direct logistical support from the Libyan Army, attacked the government military outpost of Faya Largeau in the northern desert yesterday in the latest flare-up in the Central African country's 18-year-old civil war, the government announced.

The rebel forces of former Chadian president Goukouni Oueddei, strongly supported by Libyan leader Muammar Qaddafi, and the government troops of President Hissene Habre are evenly matched with about 2,000 fighters, U.S. State Department and French diplomatic sources said. It was unclear yesterday evening which side was winning the battle for Faya Largeau, U.S., French and Chadian sources said.

The long Chadian conflict has become an area of increasing foreign policy concern for the United States and France in their efforts to contain Qaddafi's repeated efforts to bring the weak, war-wracked country under his control, according to western and African diplomatic sources.

On Wednesday, State Department spokesman Rush Taylor publicly warned that several columns of dissident troops were about to attack Faya Largeau "with the active logistic support of the Libyan Army." French President Francois Mitterrand, during an official visit to neighboring Cameroon, issued a strongly worded warning Tuesday that France would be deeply concerned about any Libyan intervention in Chad.

France and Qaddafi have been at loggerheads in sub-Saharan black Africa for more than a decade as the Libyan leader has tried to extend his influence over the region dominated by former French colonies such as Chad to create a Libyan-led, pan-Islamic, revolutionary African federation, according to western and African diplomatic sources.

Reuter reports from Chad's capital of Ndjamena quoting a government communique said the attack on the isolated military outpost 490 miles northeast of the capital began at 11 a.m. (6 a.m. EDT) Thursday.

The loss of Faya Largeau would be a serious blow to the year-old, western-backed government of Habre. Government forces reportedly were repulsed in mid-May with heavy losses when they tried to capture Ounianga-Kebif, another outpost about 60 miles north of Faya Largeau.

A Chadian diplomat in Washington said yesterday that the embassy here had not received any precise information, but the Chadian charge d'affaires in Paris was quoted by The Associated Press as saying that Libyans, Libyan-recruited foreigners and rebels loyal to ousted president Goukouni were involved in the fighting.

The charge, Ahmad Allam-mi, said the attackers were using Libyan-supplied heavy artillery and that Libyan helicopters and bombers were being used against the government troops at Faya Largeau.

A Paris-based spokesman for Goukouni's rebels denied that there is "any foreign presence in our ranks or any air support," United Press International reported. He added that "our forces are progressing at the expected rhythm." The rebels are members of a coalition group formed out of Chad's provisional Government of National Union of Transition, which Goukouni had headed.

A State Department official said yesterday that there were "no reports on" Libyan use of aircraft against the government defenders of Faya Largeau. A French diplomatic source said, however, that the Libyans did install military aircraft at an airbase in the northern Chadian Aouzou strip recently. "There were no aircraft up there a few months ago," he said.

There are about 11 factions with six private armies involved in the Chadian civil war, which first erupted in 1965. Most of the disparate forces today are aligned with Goukouni against Habre, but all have been involved in repeated skirmishes against each other. Each side, including Habre's, has been backed by Qaddafi at one time or another.

In November 1980, Qaddafi invaded Chad with several thousand troops in support of Goukouni's weak coalition government against Habre's forces. Habre retreated to Sudan, but Qaddafi withdrew his soldiers a year later when Goukouni, bowing to French, American and Organization of African Unity pressure, publicly demanded that the Libyan soldiers leave. The OAU replaced the Libyan troops with an ineffectual peace-keeping force, which later was withdrawn.