Tokyo police have raided a bookstore they say was used to recruit would-be jihadists in Japan and questioned a university student who allegedly planned to travel to Syria and join the Islamic State group.

The 26-year-old, who was on leave from Hokkaido University, admitted he was in the last stages of preparations for his journey to the war-torn country, sources at the metropolitan police department's public security bureau said.

The man told detectives he wanted to enlist with the extremist group accused of atrocities, including the recent beheadings of two US journalists and two British aid workers in Syria and Iraq, the Asahi Shimbun newspaper reported.

Police said the suspect had responded to a poster ad displayed at a used-book store in central Tokyo, offering paid work in Syria.

The "help wanted" ad reportedly encouraged people "not afraid of violence" and interested in work experience in the Middle Eastern country to ask a store worker for more details.

Police searched the store and questioned a man in his thirties allegedly connected to the ad.

Several other premises were also raided as part of the operation, as authorities probe whether behind the poster was a network of people helping Japanese nationals enter into contact with the group previously known as Isis.

The student might face charges over preparing or plotting to wage war privately against a foreign nation, which carry prison terms ranging from three months to five years.

The investigation was the first into possible support for the Islamist group in Japan.

In September, an Israeli official claimed that nine Japanese nationals have joined the ranks of the Islamic State.