The Islamic State militant group has claimed responsibility for detaining a Japanese man in Syria in a post on the Internet, Kyodo news agency reported on Tuesday.
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, which pits overwhelmingly Sunni Muslim rebels against President Bashar al-Assad, a member of the Alawite minority backed by Shi'ite militias from Iraq and Lebanon.