Syrian forces used non-lethal chemical weapons against rebels in the town of Homs in late December, according to intelligence service sources cited by French daily Le Monde on Saturday.
The weapons were used in four rockets fired on December 23, according to unnamed sources in Western intelligence services cited by the newspaper. Le Monde suggested Western powers played down an isolated event in the hope it would not be repeated.
Le Monde quoted French Foreign Ministry spokesman Philippe Lalliot as having told it that the matter had been investigated but that the checks did not produce evidence.
A French diplomatic source contacted by Reuters said Lalliot's response to Le Monde remained the official line.
Le Monde said Western powers had previously warned that recourse to chemical weapons could force them into action in a deadly conflict where President Bashar al Assad has used troops to crush an uprising now nearly two years old
"We checked this out, closely, and notably the videos that were circulating. We cannot say that combat gas or any lethal chemical products were deployed," Le Monde quoted Lalliot as having told it of the chemical weapons reports.
The diplomatic source contacted by Reuters said this was and remained the case.
The United Nations says 60,000 have been killed in the 22-month-old conflict.