British anti-terror police on Friday made an unprecedented appeal to find three girls who they believe are travelling to join militants in Syria as part of what officers said was a "growing trend".
Richard Walton, head of the Metropolitan Police's Counter Terrorism Command, said he was "extremely concerned" for the wellbeing of the girls.
Shamima Begum, 15, Kadiza Sultana, 17, and a third 15-year-old girl who is not being named at the request of her family, left their London homes on Tuesday and boarded a flight for Istanbul.
Police believe the three close friends, who all attend Bethnal Green Academy in east London, are following the example of one of their friends, who fled to join the Islamic State group in December.
"There's clearly a lot of encouragement going on by various groups in Syria to try and entice out young people, both young boys and young girls," Walton told AFP.
"By definition it's the vulnerable that are being enticed but these girls were not obviously vulnerable.
"Something, somewhere has happened to make them think it's a good idea to go to Syria."
Police believe that the three received support in planning the trip, but do not know the extent of their backing or if they have access to funding.
An estimated 550 Western women have travelled to join the militants Iraq and Syria.
Despite being banned from fighting, a study last month found that many are active propagandists for the cause on social media, celebrating the brutal violence of IS militants, acting as recruiting sergeants and even encouraging attacks abroad.
The London-based Institute for Strategic Dialogue (ISD) report said that women in IS-ruled territory were expected to marry, keep house and bear children.