French Prime Minister Laurent Fabius said today that government forces in Chad had succeeded in containing a Libyan-backed rebel offensive, but he did not rule out new military intervention by France.

The Socialist government this week placed its 2,000 troops in central and West Africa on alert following reports that the northern-based rebels had crossed into southern Chad. France has promised to help the government of President Hissene Habre defend a defensive zone, known as the red line, drawn roughly along the 16th parallel.

Speaking in a radio interview, Fabius said that a final decision on whether to send French troops back to its former African colony had not yet been made. France withdrew a 5,000-man expeditionary force in November 1984 under an agreement with Libya that was promptly broken by Tripoli, which kept many of its own troops in place.

Citing a report by Defense Minister Paul Quiles, who has just returned from a brief visit to Chad, the prime minister said that Habre's troops had "resisted well and pushed back most if not all" of the rebel forces.

In Tripoli, a spokesman for the rebels said that forces loyal to former president Goukouni Oueddei continued to hold five positions taken in this week's fighting, including the village of Ziguey, which is less than 200 miles from Ndjamena. He denied claims by the Habre government that Libyan troops were taking part in the fighting.

Both Fabius and Quiles today discussed the sudden flare-up in the 20-year-old civil war with President Francois Mitterrand. In addition to placing its troops on alert, France has stepped up military supplies to the Habre government from nearby bases in Cameroon and the Central African Republic.

About 20 French soldiers and new shipments of supplies arrived in Ndjamena today amid reports that additional French reinforcements had been sent to Bangui, capital of the neighboring Central African Republic.

According to French officials, 15 Jaguar and Mirage F1 fighter aircraft, which could be used in the event of a renewed rebel offensive, are stationed at Bangui. Libyan units in northern Chad are, however, equipped with Soviet antiaircraft missiles of the kind that shot down a French plane in January 1984.