"The offensive at Manbij is clearly being backed by a certain number of states including France. It's the usual support -- it's advisory," the official told AFP, without giving further details on the deployment.
France until now has only acknowledged the presence in the region of around 150 members of its special forces, deployed in Iraqi Kurdistan.
The SDF, a US-backed Kurdish and Arab alliance, are on the northern edge of Manbij, a strategic town held by IS that serves as a waypoint between the Turkish border and the jihadists' stronghold of Raqa.
Tabqa, another IS-held transit town which lies near Syria's largest dam, is also under attack.
France has 2,500 men in its special forces, of whom around 400 are deployed in 17 countries, mainly in the Sahel, the military said.
French Defence Minister Jean-Yves Le Drian had indicated last Friday, in remarks to a small state channel covering French politics, that French troops were helping operations at Manbij.
"We are providing support through weapons supplies, air presence and advice," he told the Public Senate channe