Photographer Laurent Van der Stockt reported that on April 13 he saw fighters "suffocating and vomiting" in the area after an apparent attack using chemical weapons.
The journalists said they had gathered witness accounts of the use of chemical weapons in a large area around Damascus.
One doctor in a rebel-held area told the newspaper that the weapons caused breathing difficulties, headaches and nausea, and could cause death if victims were not treated."The gases are used on the front on an ad hoc basis, avoiding widespread use that would easily provide irrefutable evidence," Remy wrote.
A top UN envoy said Wednesday there are "mounting reports" of the use of chemical weapons in Syria and called on Damascus to let in UN investigators.