Militants from the Islamic State extremist group claimed responsibility on the Internet for detaining a Japanese man believed to have been traveling with a rival Islamist group fighting the regime of President Bashar Assad.

A regional leader of the rival Islamic Front rebel group said the man, who left behind a passport identifying him as Haruna Yukawa, 42, had gone to report on the conflict between the two groups on Friday when he was apparently captured.

The Islamic Front was reaching out to the Islamic State to propose a prisoner exchange to secure Yukawa's release, but had not received a response, the regional leader told Kyodo News by phone.