Former US defense secretary Robert Gates warned Tuesday that the West can expect further attacks like the suicide bombing in Manchester that killed at least 22 people.
Gates, who served under both George W. Bush and Barack Obama, said Islamic State jihadists would step up international terror attacks as they lose ground in Iraq and Syria.
"As people scurry away from those sites, that doesn't mean they are defeated individually or have lost their commitment to attacking the 'crusaders,'" Gates said at a Washington conference.
"It just means they will change their tactics."
US-backed local forces have pushed IS out of almost all of their former stronghold Mosul in Iraq, and an offensive on Raqa in Syria is expected to begin in the coming months.
But the jihadists' eventual loss of this emblematic pair of cities should not be overstated.
"Just as we have seen Al-Qaeda metastasize subsequent to the killing of Osama bin Laden back in 2011... I think you will see ISIS become more active and more aggressive in a variety of places in the West," Gates predicted.
President Donald Trump has instructed the Pentagon to "annihilate" IS in a bid to prevent escaped foreign fighters from returning home.
The move to encircle then kill as many jihadists in place as possible -- rather than letting them exit a city and targeting them as they flee -- reflects an increased urgency to stop battle-hardened jihadists bringing their military expertise and ideology back to European capitals and other areas.
Twenty-two people, including children, were killed in a suicide bombing at a pop concert packed with teens in the British city of Manchester late Monday. Another 59 were wounded.
IS has claimed responsibility.