A Japanese man believed to have been captured in northern Syria by Islamic State militants said he came to the conflict-ravaged country in preparation for his plan to supply rebels fighting the Assad regime, a rebel leader said Monday.

Haruna Yukawa, 42, reportedly said he wanted to open an office in Turkey, which borders Syria, to bring supplies to rebels. Yukawa also said he planned to follow the movements of the Syrian military as well as the Islamic State, a militant group operating in Iraq and Syria, according to the regional leader of the Islamic Front, a rival rebel group to the Islamic State.

There is information that the Islamic State may still be holding Yukawa in Al-Bab, a suburban city outside of Aleppo, the rebel leader told Kyodo News in Kilis, southern Turkey, citing a tip from the Free Syrian Army, another rebel group.