The plan, which is to be set on a voluntary basis, would affect human resources, IT, and finances among others, Thierry Bodin, with the CGT union said. In addition, 80 IT jobs are to be outsourced, he said.
"This is a terrible loss of expertise. It will have consequences in terms of efficiency," Bodin said.
Sanofi had said in September it would continue to implement cost savings after having reached a 1.5 billion euros (1.34 billion pounds) cost reduction target a year ahead of expectations.
"We are blindsided as Sanofi makes significant amounts of profits. And at a time of strong social tensions in France, the government is looking the other way," Bodin said in reference to the so-called "yellow vest" protests which led to the worst riots seen in central Paris in five decades last Saturday.
A Sanofi spokesman confirmed the company's intention to shed 670 jobs out of a 25,000 workforce in France and said the plan was to be completed by the end of 2020.
At the same time, Sanofi will invest 700 million euros in France to upgrade its production sites, notably in the areas of vaccines manufacturing and other biologic medicines.
(Reporting by Matthias Blamont; Editing by Leigh Thomas and Mathieu Rosemain)