Prime Minister David Cameron vowed to defeat jihadists and defended Britain's secret services on Friday after media reports named Islamic State executioner "Jihadi John" as London graduate Mohammed Emwazi.

"We will do everything we can with the police, the security services, with all that we have at our disposal, to find these people and put them out of action," Cameron said at a news conference in Wales.

"Jihadi John," believed to be responsible for beheading at least five Western hostages, was identified Thursday as a Kuwaiti-born computing graduate who had lived in London since the age of 6.

As families of the slain hostages were calling for justice, Britain's domestic spy agency MI5 came under scrutiny following revelations Emwazi had been known to security services for several years.

"I work very closely with our security services, I meet with them regularly, and ask searching questions" Cameron said.

"While 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.

Campaign group Cage said MI5 had been tracking Emwazi, aged in his mid-20s, since at least 2009.

"MI5 blunders that allowed Jihadi John to slip the net," read a headline in the Daily Telegraph newspaper, while the Daily Mail asked, "On the MI5 watch list, so how could he escape to Syria?"