Even as the Islamic State terrorist group’s territorial footprint recedes, it remains as dangerous as ever — with the potential to launch a 9/11-style attack — acting Homeland Security Secretary Elaine Duke warned this week.

The Daily Mail reported Duke’s comments, which she delivered at the U.S. embassy in London.

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“Terrorists are strong, they are adaptable, and the terrorist threat is the highest it has been since pre-9/11,” she said, according to the British newspaper. “We have got to have every tool that’s possible.”

U.S.-backed forces this week liberated Raqqa, a Syrian city that ISIS had claimed as its capital. It was a major blow to the movement’s self-styled caliphate, which has lost more than 25,000 square miles that it had controlled in Iraq and Syria at its peak in early 2015.

“It’s going to be exceedingly dangerous to underestimate these people.”

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According to estimates by the International Centre for the Study of Radicalisation at King’s College in London, the Islamic State’s revenues have fallen by more than half since 2014.

But Duke predicted that ISIS would keep up a steady stream of “small plots” such as knifings and van attacks to maintain its visibility and draw financial contributions.

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“But [that] does not mean they’ve given up on a major aviation plot,” she said, according to the Mail.

Kyle Shideler, director of the threat assessment office at the Washington-based Center for Security Policy, told LifeZette that Duke is right to be concerned.

“It’s going to be exceedingly dangerous to underestimate these people,” he said.

Shideler said ISIS will be more dangerous, in some ways, without large amounts of land to defend.

“We do have to realize as they continue with the Islamic State — it’s not going to end, just change — we are going to have large numbers of well-trained jihadists who — they know the game,” he said.

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According to the Mail, Britain’s top spy chief also warned this week of the escalating danger of terrorism. Andrew Parker, director general of MI5, said in a speech that the United Kingdom faces threats from terrorists who are moving faster from planning to action — taking advantage of “safe spaces online” to keep their plots secret.

Shideler said American security officials need to remain vigilant in identifying potential risks. He said it does not take the resources of a quasi-nation like ISIS to carry out a major terrorist attack. He noted that the 9/11 attacks in 2001 cost roughly $250,000.

“The reality is that terrorism is not that expensive,” he said.