The Prime Minister has called on airlines and internet companies to do more to prevent British teenagers travelling to the Middle East to join Islamic State.
Mr Cameron told MPs it was "concerning" that three British schoolgirls believed to be heading to Syria had been allowed to board a flight to Turkey without being questioned.
He said 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.
Mr Cameron also said he was "horrified" by the way British teenagers appear to have been "radicalised and duped" while "at home on the internet in their bedrooms".
Citing reports that one of the girls had been following 70 known extremists online, the Prime Minister said internet companies had a "social responsibility" to cooperate with authorities in taking down extremist material and notifying authorities of contact between extremists and young people.
New footage appears to show three missing London schoolgirls in Turkey during what is believed to be their journey to Syria.
The families of three schoolgirls who fled to Syria to join IS have questioned claims they funded their journey by selling family jewellery.
Relatives of London schoolgirls thought to be in Syria tell ITV News police failed to warn them the girls were at risk of radicalisation.?