David Cameron has spoken out in defence of the intelligence and security agencies after associates of the terrorist known as Jihadi John accused them of playing a part in his radicalisation.

Speaking after the masked militant - seen wielding a knife in a series of barbaric videos of hostage beheadings - was identified as Kuwait-born 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".

Downing Street described as "reprehensible" a claim by advocacy group Cage that MI5 was partly to blame for Emwazi turning to violence, because of its alleged harassment and interrogation of the "beautiful young man".

Security services are facing mounting questions over claims that British graduate Emwazi was known to MI5 for some time before leaving for Syria in 2013 and beginning on the road to international notoriety.

Speaking in Cardiff's Millennium Stadium, Mr Cameron declined to comment directly on the Emwazi case, but said it was important for Britain to "get behind" the security services.

"When there are people anywhere in the world who commit appalling and heinous crimes against British citizens, we will do everything we can, with the police, with security services, with all that we have at our disposal, to find these people and put them out of action. That is the number one priority for me," said the Prime Minister.

"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."

He added: "In our country we do have proper ways of calling the security and intelligence services to account and scrutinising the work that they do, not least through the Intelligence and Security Committee in the House of Commons and in Parliament, which has been considerably strengthened in this parliament by this Government, with additional resources.

"So I'm satisfied we have in place the right ways of scrutinising the work that these extraordinary men and women do on our behalf, and 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.

"I think it is a moment to stand up and thank them for the work they do on our behalf."