Fabius did not give any details of where or by whom the sarin had been used but said it had taken place "several times in a localised manner".
Diplomats said the unidentified samples tested by the French laboratory had come from two locations: a suburb of Damascus and Saraqeb in the northwest of the country.
The samples from near Damascus were brought back to France by two Le Monde journalists who reported the use of chemical weapons in the area in mid-April.
UN investigators said earlier they had "reasonable grounds" to believe both sides in Syria used chemical weapons.
