Death will come in an allied air strike or at the hands of fellow terrorists, US Army spokesman Colonel Steve Warren said.
He insisted IS is “beginning to lose” under the pressure of relentless raids on its bases in Syria and Iraq.
But he also warned the war will last years – and as they face defeat Islamic extremists will commit Paris-style atrocities in the West.
Baghdad-based Colonel Warren said this would not be a sign of strength. Speaking in London he went on: “We view it as exactly the opposite.
“Because of the pressure on them, because they are beginning to stumble a bit, they are trying to prove they are not finished yet.”
He said air attacks have fed paranoia at the heart of IS – particularly since Jihadi John, the British thug in beheading videos, died in a drone strike last November.
Col Warren said: “When we killed Jihadi John we saw an immediate frenzy of activity. They rounded up their own fighters, there was a spike in executions.” He said air strikes brought “fear and paranoia” to the terror group and more than 100 IS leaders in Iraq had been killed since last summer.
In a stark message to British jihadis, he said: “You will be killed, either by your partners there in things like executions, by ground forces, or by the Coalition.”
The colonel claimed that, despite recruitment bids in IS propaganda videos, fewer foreign fighters are now heading to the Middle East.
He said: “The word is starting to get out. If you move to the Caliphate you are probably going to get killed.”
Listing the havoc wreaked on IS, Col Warren said it had lost 40 per cent of its territory in Iraq and 10 per cent in Syria. Up to 20,000 IS fighters have died in air strikes in 18 months.
The wages of jihadis have halved, from £300 to £150 a month, as air raids degrade cash-producing IS oil fields.
And desperate jihadi leaders have been forced to rely more on enforced conscription and child soldiers.
He added: “We believe they are now beginning to lose. We see them in a defensive crouch.” But he warned: “As we squeeze them in Iraq and Syria, we are going to see them look for other avenues.
“In desperation they are going to want to show the world they are still viable. One way to do that is through a high visibility attack outside of their borders.”
Showing his contempt for IS, Colonel Warren said: “These guys are disgusting.
“They have left bombs in refrigerators and toilets, they put bombs in the Holy Koran – we found that on several occasions in Ramadi.” Shashank Joshi, a senior research fellow at the Royal United Services Institute, stressed that IS, also called Isis, would not give in easily.
He said: “Isis has thousands of personnel in Libya, Egypt, Pakistan and Afghanistan. You have a serious problem that goes well beyond Syria and Iraq.”