France said on Thursday samples it had collected suggesting Syrian government forces had used chlorine gas in the country's civil war may **t prove to be conclusive and would need to be cross-checked with other information to determine its use. The Organization for the Prevention of Chemical ******* (OPCW) has opened an investigation into the alleged chlorine attacks, more than a dozen of which have been reported since April 11 in several areas. France, one of Assad's fiercest critics, was the first Western power to provide **n-lethal military aid to rebels. It has been a vocal critic of United States policy on Syria since President Barack Obama backed down from launching air strikes following suspected chemical attacks last year.