French forces firing tank-mounted cannons drove back rebel forces today, stalling the insurgents' advance toward Abidjan, Ivory Coast's commercial center.

The fighting around the western city of Duekoue was one of the heaviest engagements by French forces in the three-month-old rebellion that is splitting their former West African colony. The stepped-up French deployment may be decisive in stopping the rebel campaign to drive out President Laurent Gbagbo and seize the entire country.

The rebels have had several days of success in their push south and east toward Abidjan, one of West Africa's most vital economic hubs and ports. With so much at stake for the region if Ivory Coast falls into chaos, the French decided to take up their largest military role in years in their former colonial empire.

As rebels seized the far western city of Man on Thursday and pushed farther south Friday, French forces dug in at the western city of Duekoue, 210 miles northwest of Abidjan.

Fighters of the French Foreign Legion and other elite troops had set up antitank missiles, rocket launchers and heavy machine guns outside Duekoue to stop any rebel advance, calling their positions in the area the "stop point" for any insurgent drive on the commercial capital.

The rebels moved today to bypass the French. In hours of fighting with Ivory Coast government forces, rebels took the town of Blodi, just outside Duekoue, on a secondary road that would let them skirt the French position and push on to Abidjan. Ivory Coast's forces initially fled, leaving 40 French troops posted near Blodi to face a fast-moving convoy of rebels, French Capt. Hubert Dunant said.

The French fired two warning shots. When the rebels responded with assault rifle fire, Dunant said, French forces blasted the lead rebel vehicle with fire from a light armored tank.

The soldiers thought four to seven rebels were killed, with their bodies taken away by comrades, Dunant said.

French authorities in Abidjan described the encounter as a rebel "ambush," but rebel leader Felix Doh, a commander in the west, said rebels happened upon the French while chasing fleeing government forces.

The French said they suffered no injuries. There was no immediate information about rebel or government casualties.

Ivorian rebels took up arms Sept. 19 with an unsuccessful coup attempt but quickly seized the northern half of the country. An Oct. 17 cease-fire stalled fighting in the north, but a western front emerged in recent weeks, threatening Abidjan from another side.

Rebels in the west have not entered peace talks, and their ranks include fighters from neighboring Liberia who are known for rape and looting among civilians.