The Japanese foreign ministry said on Monday Japan was investigating whether one of its citizens had been captured in northern Syria by the Islamic State in Iraq and the Levant (ISIL) fighters.
Ministry officials could not be reached for comment immediately.
A video clip posted on YouTube this week showed a man wearing a T-shirt lying on the ground being questioned by unidentified people. The man responded by saying his name was Haruna Yukawa and that he was both a journalist and a doctor.
The name is the same as that of a chief executive of a self-described private mercenary and security firm.
More than 170,000 people have been killed in Syria's civil war, Reuters reported.