Story highlights Japanese war reporter who said he intended entering Syria has not been heard of for three weeks

Jumpei Yasuda has reported from Iraq, and had been held in custody several times, including by an Iraqi militia in 2004

Tokyo (CNN) Fears are mounting for a freelance Japanese war reporter who has not been heard from for more than three weeks, after telling a friend he was planning to enter Syria.

On June 23, Jumpei Yasuda, 41, told a close friend over the phone from Turkey of his plans to enter the war-torn country. Yasuda had previously reported from Syria.

The friend, who did not want to be named, said he believed it was highly likely Yasuda was being held by Islamic militants.

Yasuda's Twitter account , previously posted to frequently, has not been updated since June 20.

His last tweet referred to challenges to his reporting which had become "no laughing matter," and said it would be difficult to continue making updates about his work on blogs and social media in real time.

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