The UK government is urgently looking into how it can keep young girls from joining the Islamic State (IS) as three more teens are suspected of flying to Syria. 

Kadiza Sultana, 17, Shamima Begum, 15, and Amira Abase, 15, flew to Istanbul from London last week under the guise of attending a wedding, but their parents fear they will join IS.

One of the girls' friends was suspected of flying to Syria last year, and remains missing. The trio were questioned by Scotland Yard in December, but investigators did not suspect they would follow their friend. 

The teens left for Turkey on Tuesday, and have not been heard from since. 

"The message we have for Amira is to get back home. We miss you. We cannot stop crying. Please think twice. Don't go to Syria," Abase's father, Hussen, told reporters. 

The distraught dad said his daughter showed no signs of extremism. 

"She doesn't dare discuss something like this with us," he insisted. "She knows what the answer would be."

Begum's family was similarly surprised about her possible plot to join the terrorists. 

"She's a clever girl but she's only young and young minds can easily be swayed," her sister, Renu, said.

"My little sister is an A-star student. To convince such young girls at that age, who are vulnerable, it's just wrong. It's a really evil thing to do."

On Sunday, MPs debated what could be done to stop future nationals from joining IS.

Prime Minister David Cameron proposed increased screening at airports, and assistance from colleges and universities to identify susceptible students. 

"The fight against Islamist extremist terror is not just one that we can wage by the police and border control," he declared.