Silhan Ozcelik,a 17 year old kurdish girl, of Haringey, N London, who went missing a month ago and is thought to be the first UK female to volunteer against the fanatics.
Police have pinned an appeal poster to a noticeboard at the Kurdish Community Association in Haringey, north London, where she was an active member.
She was last seen in Belgium, according to Kurdish community in London.
Describing Silhan as a missing person, the poster says the teenager was last seen on the morning of October 27 at St Pancras international rail station.
Officers think she caught a train to Belgium and then travelled to Syria probably via the Turkish border.
It is possible that Silhan, who is a high achiever at school and hoped to become a human rights lawyer, wanted to join the YPJ, an all-female militia protecting Kurds from the ISIL terrorists. However, according to her family, she went to Syria simply to carry out humanitarian work.