A top US general today warned the West should be braced to spend “blood and treasure” in a “generational struggle” against ISIS.

Former spy chief General David Petraeus said while coalition forces could target extremists in one war zone, they would simply surface on another battlefield.

Speaking to defence think-tank the Royal United Services Institute, he called for the crazed jihadists - who face defeat in Syria, having been driven out of Iraq - to be targeted in every lair.

“You have to go after them wherever they are,” he told military experts.

The retired Army chief gave a keynote speech to the defence and security think tank (
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Getty)
Militants must be taken down wherever they pop up, Gen Petraeus said (
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Getty)

“You cannot play whack-a-mole - you have to whack all the moles all the time everywhere they might be.

“You have to try to prevent ungoverned or inadequately governed spaces emerging in the Muslim world.”

While allied forces would eventually “put a stake through the heart” of ISIS leader Abu Bakr al-Baghdadi, tackling the “virtual caliphate” would be more difficult, warned the ex-CIA director and past commander of US Central Command and of coalition forces in Afghanistan and Iraq.

Abu Bakr al-Baghdadi will eventually get "a stake through the heart" (
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Reuters)

Gen Petraeus warned: “We will defeat the army that is the Islamic State and at some point we will put a stake through the heart of Baghdadi, the leader of the Islamic State.

“We are not going to be able to put a stake through the heart of the virtual caliphate; this is the ideology that’s being propagated on the internet.

RAF Tornados spearheaded Operation Shader, the military codename for the fight against ISIS (
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PA)
RAF Typhoons pounded the terrorists' lairs in Syria and Iraq (
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Getty Images Europe)

“This is an enormous topic that needs to be addressed.”

In a gloomy assessment of the ongoing threat from the Islamic extremists, he said: “This is a generational struggle, this is not the fight of a decade or a few years where you take a hill, fly a flag and go home to a victory parade.

“This is going to be a generational endeavour, and if you’re engaged in something that long you will obviously have to have a sustained commitment, and to be able to sustain the commitment it has to be sustainable - that is, measured in blood and treasure.”

He also called for a crackdown on hate-filled jihadi videos online, in the same way web giants targeted child porn.

Gen Petraeus called for a crackdown on cyber jihadis (
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PA)

And the retired chief branded cyber warfare “a whole new battle space - very much the equal of ground, sea, air, sub-sea, space and now cyber space”.

Gen Petraeus added: “Battles are already being fought in cyber space, constantly.”