Police scramble to find teens

Six days ago, the trio of British girls ran away from home and flew from London to Istanbul. It's unclear where 15-year-olds Amira Abase, Shamima Begum and 16-year-old Kadiza Sultana are.

But authorities worry their next stop will be Syria.

"This is an ongoing, live counterterrorism operation, and we're concerned about these three girls," London Metropolitan Police Cmdr. Richard Walton said.

"There's a good possibility, a strong possibility, that we can stop them going into Syria. And our main objective here is to make contact with these three young girls, for them to make contact with us, preferably."

The notion that three girls from London could succumb to the lure of ISIS highlights the terror group's aptitude for recruiting young foreigners.

British Prime Minister David Cameron said the case illustrates the need to fight ISIS on multiple levels.

"It does make a broader point which is the fight against Islamist terror is not just one that we can wage by the police and border control," Cameron said Saturday.

British police say they are concerned about the number of young women and girls who are either hoping to or who already have joined ISIS. While they wouldn't divulge numbers, they say it's a growing trend.

British police say once they're in Syria with ISIS, it's unlikely they will ever be allowed to leave.