There's mounting concern for three teenage British girls who've skipped the country to join Islamic State.

The girls have taken a Turkish Airlines flight to Istanbul, and police fear they're planning to cross the border into Syria to join ISIS.

Mussurut Zia of the Muslim Women's Network fears the girls don't realise what they're letting themselves in for.

She says they've grown up with the concept of having equal rights to men.

"I don't think they've known for one instant what the reality is for them and I really don't doubt that they will be used for the comfort of men or as jihadi brides."

She says it's important to pinpoint what ISIS is doing that appeals to vulnerable girls.

"What is here that is missing here at home, that can be fulfilled overseas when according to them they go over to fight?"