The global chemical weapons watchdog says members of a fact-finding mission investigating allegations of chlorine attacks in Syria were ambushed and detained on Tuesday before being released.

The Organization for the Prohibition of Chemical Weapons released details Wednesday of the ambush in rebel-held territory, saying it started when one vehicle in the joint OPCW-United Nations convoy was severely damaged by a roadside bomb.

The remaining vehicles retreated to a safer location when they were ambushed and the lead vehicle sprayed by automatic gunfire.

The OPCW said occupants of two remaining vehicles "were briefly detained by some gunmen" but were released after intervention by Syria's main opposition group.

One driver sustained minor injuries in the attack, the OPCW says. The rest of the team is back in Damascus.