A preliminary report by the world's chemical weapons watchdog says "various chlorinated chemicals" were found at the site of an April attack in Douma, Syria, that killed dozens of civilians and prompted airstrikes by Britain, France and the United States.

The Organisation for the Prohibition of Chemical Weapons (OPCW) sent a fact-finding mission to Douma roughly a week after the April 7 attack in the enclave near Damascus.

"Along with explosive residues, various chlorinated organic chemicals were found in samples from two sites," it said in Friday's report, indicating that chlorine may have been used as a weapon.