Fighting in Chad has reached a "critical phase," in the words of one U.S. official, as government troops step up their efforts to stop the rapid advance of rebels backed by Libya.

U.S. officials fear the Libyan government of Muammar Qaddafi is using the civil war to establish a new beachhead against U.S. allies in northern Africa.

The Chadian government troops have received about 200 tons of French supplies, including mortars, antitank weapons and rifles, that were flown in last month under a military cooperation agreement France has with its former colony. France also has moved to help the government of President Hissene Habre by sending in military advisers, according to a Defense Department official and a published report in France.

The French advisers went into Chad without uniforms and have been ordered to stay out of the fighting, the well-informed daily Liberation reported Saturday in Paris. But the U.S. official said some of the "several dozen" advisers had moved out of the capital of Ndjamena and were with the forces loyal to President Habre. They came into Chad after the rebels made their first attack on the government garrison at Faya Largeau on June 23, the official added.

France has denied strongly that it is sending any military advisers to aid the Chadians, although French officials admitted that several civilian advisers went to Ndjamena to help deliver the arms. Chad's ambassador to Washington, Mahamat Ali Adoum, said those civilian advisers have returned to Paris.

Chad twice has requested direct French help in the form of more military supplies and troop involvement. But this latest flare-up in the civil war that began in 1965 has presented public opinion problems for the French government of President Francois Mitterrand. Despite its military cooperation agreement with Chad, France does not have a security pact that obliges it to use troops to protect Chad's government, and the current fighting between Habre and Goukouni is part of a seesaw battle that has seen both men hold power there with support from France.

Over the years the civil war has involved about 11 factions with six private armies. Goukouni, at least for now, has collected most of the disparate forces under his banner as he seeks to oust Habre, but in the past all have been involved in repeated fights against each other.

For part of Goukouni's term as president, Habre served as defense minister. Then he moved into the opposition and irked the French government by actions such as kidnaping a French doctor for ransom before finally driving Goukouni from power 13 months ago.

The rebels' success, however, has frightened some of Chad's neighbors who look to France, as the former colonial power, for defense aid.

The United States also is watching closely the latest fighting in the mineral-rich nation that is nearly twice the size of Texas.

"The U.S. is strongly opposed to the flagrant intervention of Libya with its neighbors," State Department spokesman Michael Wygant said.

Qaddafi, who one well-informed official characterized as a "rogue elephant" seeking to destablize the region, has long been considered by Washington as a Soviet proxy who threatens U.S. allies, such as Egypt and Sudan, as he seeks a revolutionary African federation. But U.S. officials have ruled out the use of U.S. arms or troops to support the Chadian government forces fighting Qaddafi's allies.

The State Department said the Libyans are providing crucial "logistical and adminstrative and technical assistance" to Goukouni's forces. Officials have suggested that the Libyans could also be maintaining and firing heavy weapons for the rebels.

State and Defense department officials said Goukouni's forces have artillery that they are not capable of operating by themselves.

Chad's information minister, Soumalia Mahamat, said Tuesday that government forces had retaken the important city of Abeche and seized important quantities of arms from rebel forces led by former president Goukouni Oueddei, The Associated Press reported from Ndjamena, the capital. The Habre government was also bolstered by the announcement that Zaire would send 1,750 more troops and three Mirage fighters to Chad, AP reported, quoting Zaire's official news agency.