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LYON, France — Weeks after President Donald Trump abruptly announced that the U.S. would withdraw troops from Syria because "we have defeated ISIS," one of the world's top law enforcement officials warns that the terrorist group is still an immediate threat.

ISIS fighters retain the expertise, networks and intention to strike at the United States, Jürgen Stock, the secretary-general of Interpol, told NBC News in an exclusive interview at the organization’s headquarters in France.

"The threat is still very acute, the threat is complex and the threat is more international than ever,” Stock said.

Trump announced the Syria withdrawal on Dec. 19, a decision that has drawn much criticism and pushback from leaders in his own party.

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His plan has also caused consternation among many countries participating in the coalition to defeat ISIS, as they fear a U.S. withdrawal would create a power vacuum allowing ISIS to become resurgent and making it more difficult to capture remaining ISIS sympathizers in Syria.

Next week, a draft Pentagon report will warn that without continued pressure, ISIS could regain territory in six to 12 months, two U.S. officials familiar with the draft told NBC News.

Interpol's chief also warned that there were an unknown number of former ISIS fighters now detained in Syria, whose activities and connections need to be investigated as thoroughly as possible before they are potentially released.

US-backed Kurdish forces have previously said that they hold about 2,700 foreign ISIS members and their families.

Among the chief causes of concern for global counterterrorism investigators is the hardened core of fighters of foreign origin who have traveled through areas once controlled by ISIS.

Many of these individuals are “battle-hardened” and possess specific skill sets such as bomb-making that could be used in terror attacks in the future, Stock said.

“Foreign terrorist fighters from more than one hundred countries have been traveling into the conflict zone in Syria and Iraq, so there is a big network now of contacts,” Stock said. “They are in contact with each other, they can share experience, they can talk about attacks that they are going to plan.”

The primary focus for terror attacks remain “soft targets” in areas with unstable security environments, Stock said. “It might be bars, it might be restaurants, it might be hotels as we have seen very recently in Nairobi … places that are favored by the Western community.”