France announced today that it will step up arms shipments to its former African colony of Chad following a new offensive by Libyan-backed rebels against government-held positions.

France's Socialist government underlined its concern about the sudden flare-up in the 20-year-old civil war in Chad by sending Defense Minister Paul Quiles to Ndjamena this evening for urgent talks with President Hissene Habre. According to reports from the Chadian capital, the latest fighting took place south of the 16th Parallel, which Paris has pledged to defend against the northern-based rebels.

The Chadian Embassy in Paris announced tonight that "Libyan troops" had attacked a government outpost at Ziguey, south of the 15th Parallel and less than 200 miles from the capital. The report could not be confirmed immediately.

"By this offensive, [Libyan leader Muammar] Qaddafi is defying the people of Chad, the countries of Africa, and the friends of Chad," Chadian Ambassador Allam-Mi Ahmad said in a statement urging France to respond to "Tripoli's challenge."

France sent about 3,000 soldiers to Chad in August 1983 to support Habre's government but withdrew them the following year under a mutual troop withdrawal agreement with Libya. American satellite photos subsequently showed that Qaddafi's government had reneged on the deal and was keeping a sizable force in northern Chad.

The latest rebel offensive follows a logistical buildup by forces loyal to former president Goukouni Oueddei along the "red line," as the French call the security zone at the 16th Parallel. Earlier this month, the Libyans flew half a dozen French journalists to Chicha to see the rebels' new military hardware, which included light airplanes, heavy mortars and antiaircraft guns.

The announcement of accelerated French supplies to the government side was made by President Francois Mitterrand's adviser for African affairs, Guy Penne, who was dispatched to Ndjamena earlier this week for talks with Habre. France already supplies the Chadian government with most of the arms it requires for its 5,000-man Army.

Speaking in Libreville, Gabon, Penne described the latest outbreak of fighting as "very serious and very important," adding that France would respond "appropriately."

With little more than a month to go before crucial legislative elections in France, political analysts here believe that Mitterrand would be very reluctant to send French troops to Chad for the second time in his presidency. The landlocked nation, which ranks as one of the world's poorest, has created political and military problems for a succession of French governments.

Chadian radio announced today that the rebel forces had attacked government-held positions at the town of Koro Toro on the 16th Parallel but had been pushed back. The government earlier said it had retaken the garrison town of Kouba Olanga on Tuesday morning, killing or capturing hundreds of rebels.

Fighting was reported continuing around the town of Oum Chalouba in the northeast, but there was no immediate word of the outcome.

The Chadian government has claimed that several thousand Libyan troops are fighting alongside the rebels. The rebels have denied active Libyan involvement in the fighting, while acknowledging that they depend on Tripoli for most of their logistical support.

The French journalists who visited northern Chad earlier this month reported evidence of close Libyan supervision over Goukouni's troops, who included 16-year-old boys. The journalists, accompanied by a Libyan colonel who identified himself as a "liaison officer" with Goukouni's forces, were not allowed to visit the town of Faya-Largeau, which is believed to be an important rebel military base.

At the annual Franco-African summit in Paris last December, Mitterrand warned Qaddafi against mounting a military operation in southern Chad. France was reported to have resumed reconnaissance flights over Chad late last year in response to signs of the Libyan military buildup in the north.