Airlines and internet companies need to do more to prevent radicalised British teenagers travelling to the Middle East to join radical group Islamic State, David Cameron has said.
Mr Cameron told MPs he was "horrified" over by the case of three London girls believed to have travelled to Syria to join IS after being indoctrinated by extremists online.
He announced that Home Secretary Theresa May and Transport Secretary Patrick McLoughlin will be talking with airlines on new "proportionate" arrangements to ensure that children who are at risk are properly identified and questioned, and said border police should be alerted of any concerns so they can stop individuals from travelling.
And he said that internet companies must live up to their "social responsibility" by taking down extremist content and improving co-operation with the authorities over contacts between extremists and young people vulnerable to radicalisation.
Shamima Begum, 15, Kadiza Sultana, 16, and 15-year-old Amira Abase are currently being hunted in Turkey after they boarded a flight from Gatwick Airport to Istanbul last Tuesday.
The headteacher of the girls' school said he was "shocked and saddened" by the girls' disappearance, but said that police had not found evidence that they were radicalised at the school.
Mark Keary, principal of Bethnal Green Academy, east London, said police spoke to the girls after another student disappeared in December and indicated at the time that there was no evidence that they were at risk of being radicalised or absconding.
He also said that access social media at the school is "strictly regulated". A tweet sent from a Twitter account under Shamima's name was sent to Aqsa Mahmood, who left Glasgow for Syria to be a "jihadi bride" in 2013.
In a statement to the House of Commons, Mr Cameron voiced concern that unaccompanied teenagers were able to board a flight to Istanbul without the airline asking questions.
The Prime Minister told MPs: " All of us have been horrified by the way that British teenagers appear to have been radicalised and duped by this poisonous ideology of Islamist extremism while at home on the internet in their bedrooms.
"They appear to have been induced to join a terrorist group that carries out the most hideous violence and believes girls should be married at nine and women should not leave the home.
"Their families are understandably heartbroken - and we must do all we can to help."
Mr Cameron said that the problem was " not just an issue for our police and border controls", and that everyone from educational institutions to religious leaders and families had a duty to help prevent young people being radicalised by IS - also known as Isis or Isil.
He added: "Stopping travel to join Isil is vital ... W hat this incident has highlighted is the concerning situation where unaccompanied teenagers like these - who are not a known risk - can board a flight to Turkey without necessarily being asked questions by the airline.
"We need new proportionate arrangements with airlines to ensure that these at risk children are properly identified and questioned - and the Home Secretary and Transport Secretary will be working with the airlines to bring this about.
"Whenever there are concerns, police at the border should be alerted so they can use the new temporary passport seizure powers to stop people travelling."
Mr Cameron said that one of the girls was reported to have been following as many as 70 extremists online, underlining the importance of work being done by the authorities with social media companies.
" We have made progress with these companies who are working with the police and Home Office to take down extremist content online. And at the EU council we agreed to do this across the European Union," said Mr Cameron.
"But we also need greater co-operation over contacts between extremists and those who could be radicalised. Internet companies have a social responsibility. And we expect them to live up to it."
The PM also said that the UK would continue to press for police and security services to have access to airline passenger name records - including details of bank accounts and credit cards used to buy tickets - for as many routes as possible in and out of Britain.
Following extremist murders in Paris and Copenhagen, EU leaders have agreed to adopt a "strong and effective" directive on the issue as a matter of urgency, he said.
Relatives of the three schoolgirls have made emotional pleas for them to come home amid fears they may have been recruited by jihadists on the internet.
At the Bethnal Green Academy today, police officers spoke to staff, while pupils attended assemblies with support agencies and a team is on site to speak to students and teachers.
In a statement issued at the school, Mr Keary said: "We are all shocked and deeply saddened by the news that three of our students have been reported missing from home, they boarded a flight to Istanbul from Gatwick Airport last week, and our thoughts are with the families of the missing girls at this time.
"This situation follows an earlier disappearance of a student in December of last year.
"The police spoke to that student's friends at the time and further to this, they indicated that there was no evidence that the girls were at risk of being radicalised or absconding. The police have also confirmed that the families of the missing girls were unaware of their plans to leave the country."
He said students are unable to access Twitter or Facebook on school computers.
"With such measures in place, police have advised us that there is no evidence that radicalisation of the missing students took place at the academy," he said.
Mr Keary added: "From the increasing number of similar news reports around the world, it is clear that this is an international issue which is increasing in severity and it's affecting schools across the country and beyond. We are constantly reviewing our safeguarding procedures."
The school is supporting the police investigation, he said, adding that police have requested that they make no further comment as the inquiry is ongoing.
Mr Keary said the school, which has been rated as "outstanding" by Ofsted, is "extremely proud" of the "exceptional learning experience" it provides to students.
"A core aspect of our ethos is to promote the British values of democracy, tolerance and respect," he said. "Particularly respect for other cultures and this is taught through a wide variety of curriculum topics and learning programmes.
"We also operate an outstanding system of pastoral care and personal support, which aims to ensure that all students are comfortable addressing any issues or concerns to members of staff."
Mr Keary reiterated the police appeal, adding: "Our thoughts are with the families of the missing girls as we await news and hope for their safe return to their homes and families."
Scotland Yard confirmed last week that the girls were spoken to in December following the disappearance of their friend but said there was nothing to suggest at the time that they were at risk.
Officers are in Turkey, though a Metropolitan Police spokesman refused to confirm whether they were involved in the search for the girls.
Shadow home secretary Yvette Cooper said: "There are serious questions about why Turkish Airlines allowed three 15 and 16-year-old girls unaccompanied onto a flight. Why were such young people allowed to fly on their own in the first place? And why - when we know other young people seeking to travel to Syria have initially travelled to Turkey - did the airline not raise immediate concerns to the police and security services?
"The Government had told us they were already working with the airlines to disrupt people travelling via Turkey to Syria so it is extremely hard to understand why it has taken until now for the Home Secretary and the Transport Secretary to contact Turkish Airlines about this."
Ms Cooper accused the Prime Minister of resisting supporting community-led projects to counter radicalisation through the Prevent programme and said that the Department of Education was not taking enough action on the issue in schools.
And she added: "Given the clear evidence these girls had been contacting other Isil recruits on Twitter, we also need a Government strategy for counteracting radicalisation on social media."