Jean-Marie Guehenno, the visiting head of the International Crisis Group,  said it was  "interesting to see how extremism leads to Islamism, rather than Islam leads to Islamism".

"Many of the terrorists of today would, 30 years ago, have joined the Baader-Meinhof Gang or the Red Brigades, because revolutionary Marxism was the  way to fight violently the established order" he told the Lowy Institute in Sydney.

"Today they [rally under] the flag of international jihadism.. .. You are a lost soul in some derelict suburb in France or elsewhere and by joining that fight [in Syria or Iraq] your violence acquires a whole new dimension."

Mr Guehenno, a onetime French diplomat who is also a former head of UN peacekeeping,  said French police estimated  one in five French jihadis who joined Islamic State in the Middle East were converts to Islam.

"They became Islamists because it was a way to join a violent fight, not because religion is their primary motivation," he said on Friday. "To understand that,  it is better to have a degree in psychology than a degree in politics."

The Red Brigades in Italy and the German Baader-Meinhof  group were militant ultra-left groups which conducted a string of politically-motivated kidnappings, bombings and murders in Europe in the late 1960s, 1970s and 1980s.

Mr Guehenno said the struggle against Islamic State and its spin-offs was an "information war" with social media among its key  battlegrounds.

"The horror of limited acts  and the use of videos... replaces mass killings.;.. it is not thousands of people being killed, but its so atrocious that it has global repercussions," he said.

"They know that action triggers reaction," he said, adding that the challenge was to "calibrate our reactions".

Surprise was the "new normal" in international affairs because of the "convergence of local grievances with global agendas being the key factor in triggering major catastrophic events", Mr Guehenno told the Lowy Institute.