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.

If the man is in the hands of Islamic State leaders, they will probably demand a ransom from the Japanese government. If he has been captured by low-level fighters, his fate will be less certain, the regional leader said.

Yukawa reportedly told the militants he had wanted to open an office in Turkey, which borders Syria, to bring supplies to the rebels. According to the Islamic Front leader, he also said he had planned to follow the movements of the Syrian military and the Islamic State, a militant group operating in Iraq and Syria.

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On Aug. 14, Yukawa was with fighters from the Islamic Front and the Free Syrian Army in Marea, about 30 km north of Aleppo, when fighting broke out against the Islamic State, according to an FSA officer.

The Japanese national was likely left behind as the party retreated, and may have mistakenly run toward enemy fighters in the confusion, the FSA officer said.

The Islamic Front regional leader originally said Yukawa was captured on Aug. 15, but he said later the date was actually Aug. 14.

A statement posted Monday online claimed Islamic State intelligence officers had captured a Japanese spy named Haruna Yukawa, but did not disclose Yukawa’s current condition. The statement’s authenticity could not be independently verified.

Vice Foreign Minister Akitaka Saiki told reporters Tuesday morning in Tokyo that the Foreign Ministry had received no new information and that the government was working to find out more. Saiki declined to comment on the purported Islamic State statement.

The man believed to be Yukawa apparently left a passport and three pieces of paper at an Islamic Front base outside Aleppo, northern Syria, the rebel leader said. Although incomprehensible in places, the pieces of paper carry messages written in English, Arabic and Japanese that appear to explain his intention to aid forces fighting to bring down Assad.

According to the Islamic Front’s regional leader, Yukawa flew from Istanbul to Gaziantep in southern Turkey on July 27 and entered Syria the following day. Yukawa said he was robbed of all his cash on the way, the person said, adding that the Islamic Front had been providing him with food and shelter and that he had since converted to Islam.

In the weeks that followed, he is said to have gathered information on militants fighting the Assad regime and on the deadly barrel bombs being used by Syrian government forces. Yukawa also purportedly said he wanted to write a report on the Islamic State.

Yukawa and several Islamic Front fighters were captured in Marea, about 30 km north of Aleppo, after the group, fearing that Islamic State militants were surrounding them, decided Friday to launch an attack, the regional leader said. Thirteen Islamic Front fighters were injured.

There have been unconfirmed reports that Yukawa has been moved from Marea.

Hiroyuki Aoyama, an expert on Syrian politics and a professor at Tokyo University of Foreign Studies, said Yukawa’s capture by the Islamic State might have been an accident.

“The truth may be that an enemy they captured in regular fighting happened to be Japanese,” he said.

On a video clip posted on YouTube the man identifies himself as Haruna Yukawa and says he is from Japan.

Asked why he is there, the man replies “a photo job,” to which his interrogator replies, “A photographer don’t dress like this.”

The man is repeatedly asked why he is carrying a gun and if he is a soldier. The man says he is “no soldier” and that he is “half journalist, half doctor,” to which the interrogator replies, “And half soldier? There are only two halves.”