(CNN) Amira Abase told her father she was going to a wedding. Instead, the 15-year-old British girl hopped on a plane with two friends to Turkey -- and police fear they are headed to Syria to join ISIS.

"What she's doing is completely nonsense," said Amira's father, Abase Hussen.

He implored Amira to come home.

"Remember how we love you. Your sister, brother, they cannot stop crying."

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

But authorities worry their next stop will be Syria

Ibrahim Kalin, a representative for Turkish President Recep Tayyip Erdogan, said at a news conference that Turkey is helping search for the girls.

"About these three British citizens: British authorities and Turkish authorities, our security forces and our intelligence service are working together diligently to find them, to stop them or to figure out what is motivating them if they came here," he said.

British authorities hope the trio will reach out to them. "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."

But so far, silence.

A knack for recruiting

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

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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.

"It needs every school, every university, every college, every community to recognize they have a role to play, we all have a role to play in stopping people from having their minds poisoned by this appalling death cult."

'You are not in any trouble'

Shamima's family has also made a public plea in hopes the girls will return.

"Syria is a dangerous place and we don't want you to go there. Get in touch with the police and they will help to bring you home," the girl's family said in a statement released by police. "You are not in any trouble."

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The family said Shamima has strong desires to help those she believes are suffering in Syria.

"You can help from home. You don't have to put yourself in danger," her family said. "Please don't cross the border. Please come home to us. Our Mum needs you home and is really worried. We are not mad at you, we love you."

Stopping the cycle

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.

Shamima's sister, Renu Begum, described the 15-year-old as an "A-star student." But even bright, promising children can fall prey.

"To convince such young girls at that age that are so vulnerable ... it's just wrong," Begum said. "It's a really evil thing to do. You're breaking up entire families."

Walton said there are ways of helping detect whether teens may be drawn to a terror group.

"If young people are showing interest in passports, trying to get hold of lots of money, there are signs. Anything breaking the normal routine, any evidence ... of children lying about where they may be," he said.

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