The Islamic State of Iraq and Syria militant group has claimed responsibility for detaining a Japanese man in Syria in a post on the Internet, Kyodo news agency reported on Tuesday.
The Japanese foreign ministry said on Monday Japan was investigating whether one of its citizens had been captured in northern Syria by ISIS.
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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 Shia militias from Iraq and Lebanon.
In a separate video posted online, ISIS warned the United States it will attack Americans "in any place" if the raids hit its militants.
The video, which shows a photograph of an American who was beheaded during the U.S. occupation of Iraq and victims of snipers, featured a statement which said in English "we will drown all of you in blood."