Three east London teenagers who went missing on Tuesday are believed to have travelled to Turkey in the hopes of crossing the border into Syria, police said today.

The girls, pupils at Bethnal Green Academy, flew from Gatwick to Istanbul on February 17. Two have been named as Shamima Begum, 15, and Kadiza Sultana, 16. The third girl, a 15-year-old, has not been identified at the request of her parents.

An appeal has been launched by police. Commander Richard Walton, of London police's Counter Terrorism Command, said that the force is "extremely concerned" for the girls' safety, as well as the increasing number of young women who have travelled to join the Islamist militants abroad, or have expressed interest in doing so.

"We are reaching out to the girls using the Turkish media and social media in the hope that Shamima, Kadiza and their friend hear our messages, hear our concerns for their safety and have the courage to return now, back to their families who are so worried about them," Walton said in a statement.

He added that Syria is "extremely dangerous" and that life for women there is often very restricted. "It is not uncommon for girls or women to be prevented from being allowed out of their houses or if allowed out, only when accompanied by a guardian.

"The choice of returning home from Syria is often taken away from those under the control of Islamic State, leaving their families in the UK devastated and with very few options to secure their safe return."

Walton expressed hope that the teenagers would be found in Turkey before they are able to cross the border. They are reported to be friends with another 15-year-old who travelled to Syria in December.

Earlier this month, Bristol schoolgirl Yusra Hussein, allegedly contacted friends for the first time since her disappearance last September to tell them she has become a jihadi bride.

Hussien boarded a flight from Heathrow to Turkey on September 24 with another teenager. It was thought that the two were attempting to enter Syria across the Turkish border to join IS militants.

Security sources revealed that Hussein had contacted the 'Jihadi Matchmaker' Twitter feed, which offers to help "link up those seeking marriage in Syria in a halal manner" before she left.

Her friends told the BBC that Hussein sent them a message on social media to confirm that she was now married in Syria.

Two other British teenagers, twins Zahra and Salma Halane who fled to Syria to marry jihadists, recently posted photos of themselves on Twitter holding guns.

"Fun day training for self defence in the Islamic state with humble sisters," the tweet, posted from an account believed to belong to Zahra, read.

Around 600 Brits and 150 Americans have travelled to Syria to join Islamist militants, and there is concern that many young people are being "groomed" by jihadists online.

A US Imam working to protect young Muslims from travelling to the Middle East says the pressure they are under is relentless.

Imam Mohamed Magid told the New York Times that he has managed to persuade five young men to abandon plans to join the jihadi cause, but has struggled in the face of constant pressure from IS recruiters.

Speaking of one young man in particular, Imam Magid said: "The recruiters wouldn't leave him alone. They were on social media with him at all hours, they tweet him at night, first thing in the morning. If I talk to him for an hour, they undo him in two hours."

Another Imam, Suhaib Webb, told the newspaper that IS' message is attractive to many young Muslims who feel ostracised by their own communities. "ISIS says: 'Come here. We've got ripped warriors,'" he said.

"It's a very simplistic response, but it's somewhat effective."

The executive director of Muflehun, a Muslim community organisation that works to prevent radicalisation, Humera Khan, has warned that it's difficult to predict who will be drawn to IS because there are "no patterns, and that's making it harder for everyone".

"They can come from every ethnic, socioeconomic group, any geographic area. But they are more often men than women, and they're getting younger."

"It is scary no matter how you look at it," she said in September.