French carmaker Renault said workers needed to make concessions on pay and conditions to safeguard the future of domestic plants.
Gerard Leclercq, Renault's head of French operations, said on Wednesday that without a deal the company would be forced to take other action to cut costs.
"If there is no deal, it is impossible to leave things as they are," Leclercq said in a BFM radio interview. "All options are open. But, for the moment, there is no plan B."
Renault, which last week unveiled plans to cut 7,500 French jobs over four years, has denied union accusations it had threatened to close two plants during the closed-door talks.
Unions were set to carry out a series of production stoppages at Renault sites around the country on Wednesday in protest at the company's demands.
On Tuesday, the carmaker offered commitments to build an additional 80,000 vehicles annually in France for Japanese alliance partner Nissan <7201.T> and German group Daimler in return for a labour deal.
The company, which is seeking a longer workweek and increased flexibility from workers, has previously offered to guarantee French plants as part of an accord, a negotiating stance denounced by some unions as "blackmail".
The rising industrial tensions at Renault prompted the government, its biggest shareholder with a 15 percent stake, to call upon both sides to negotiate in good faith on Tuesday.
The Renault operations chief said it was "perfectly legitimate" for workers to express their concerns.
"But we need to stay focused on the commitments we've made and want to obtain in the framework of a deal," Leclercq said, "precisely to avoid any French site closures."
(Writing by Laurence Frost; Editing by Mark Potter)