David Cameron says "lessons need to be learned" from the case of three British schoolgirls feared to have entered Syria to join Islamic State.
Responding to a question from the Labour MP for Bethnal Green and Bow, where the three teenagers came from, Mr Cameron said the UK needed to "tighten arrangements" at airports and borders.
He also called on schools, parents, universities to help fight the "poisonous ideology" of extremist groups such as Islamic State.
The Prime Minister also dismissed claims by the Turkish deputy prime minister that British authorities took three days to notify Turkey of the girls' movements as "not accurate".
Boris Johnson has accused advocacy group Cage of amounting "an apology for terror" after it suggested MI5 may be partly to blame for Mohammed Emwazi turning to violence.
"It was incredible that people could stand up and pretend that somehow it was the fault of the security forces for trying to apprehend and impede these guys and that that could somehow cause them to be radicalised," the Mayor of London said.
"I think that is beyond satire and amounts to nothing less than an apology for terror. I hope they will be rethinking their position."
Downing Street earlier said Cage's claim was "reprehensible".
The man identified as the Islamic State militant known as "Jihadi John" was a "popular" schoolboy who loved Manchester Utd and S Club 7.Read the full story ›
David Cameron has defended the intelligence services amid accusations they allowed the Islamic State terrorist known as "Jihadi John" to leave for Syria.
While not commenting specifically on the identity of the masked militant, understood to be Londoner Mohammed Emwazi, Mr Cameron said his "number one priority" was to ensure that those responsible for crimes against British citizens were hunted down and "put out of action".
The Prime Minister praised the work of the "extraordinary" security services, who he said made "incredibly difficult judgments" on the country's behalf.
"I work very closely with our security services. I meet with them regularly, I ask them searching questions about what they do and in my almost five years experience as Prime Minister, I think they are incredibly impressive, hard-working, dedicated, courageous and effective at protecting our country.
"All of the time, they are having to make incredibly difficult judgments and I think basically they make very good judgments on our behalf, and I think whilst we are in the middle of this vast effort to make sure British citizens are safe, the most important thing is to get behind them.
"All I can say is, even in the last few months, their dedication and work has saved us from plots on the streets of the United Kingdom that could have done immense damage."
David Haines' daughter tells ITV News families of IS victims will not get closure until there is a "bullet between the eyes" of Jihadi John.Read the full story ›
CAGE, the UK-based advocacy group who had contact with Mohammed Emwazi, the man identified by US security sources and US journalists as Mohammed Emwazi, are holding a press conference on their dealings with him. UK Editor Rohit Kachroo Kachroo reports.
The man they knew, who became Jihadi John was 'kind, gentle and softly spoken' says spokesman
It's difficult to see how the man they knew went on to become the ISIS executioner known as 'Jihadi John'.
Describing "roughing up in Tanzania" and questioning by MI5 in Amsterdam.
"He was such a beautiful young man - it's hard to believe the trajectory"
Asim Qureshi, a research director from advocacy-group CAGE, said the Mohammed Enwazi that he worked with was kind, generous and gentle.
Qureshi said the Washington Post recorded his comments incorrectly, and that he said there was a striking similarity between Enwazi and 'Jihadi John', but did not say for certain. He said:
"While I think there are striking similarities, I can't be 100% sure certain. He has got a hood on, come on guys, the guy has a hood on his head."
The widely reported identity of so-called "Jihadi John" is accurate and correct, researchers say.
The White House and Downing Street have so far refused to officially confirm whether Mohammed Emwazi is the Islamic State militant featured in a series of brutal beheading videos.
A statement from the International Centre for the Study of Radicalisation (ICSR) at King's College London said: "We believe that the identity and name published by the Washington Post and now in the public realm, to be accurate and correct."
The statement added: "British fighters have clearly demonstrated that they are not in this conflict to take a back seat. They are full participants in this war, operating as suicide bombers, hostage takers, and executioners."
The University of Westminster says it is "shocked and sickened" by claims a former student is the Islamic State militant known as "Jihadi John".
The extremist featured in videos showing the execution of Western journalists and aid workers has been identified in media reports as Mohammed Emwazi.
Reports suggest Emwazi was radicalised after leaving the University of Westminster.
A spokesperson said: “A Mohammed Emwazi left the University six years ago. If these allegations are true, we are shocked and sickened by the news. Our thoughts are with the victims and their families.
“We have students from 150 countries and their safety is of paramount concern. With other universities in London, we are working to implement the Government’s Prevent strategy to tackle extremism."