It simply would not be possible to shut down areas of the Internet that terrorists use to conduct malicious activity, the head of U.S. Cyber Command told a Senate panel on Tuesday.

"In a very simplistic way, people ask why can't we shut down that part of the Internet. ... Why are we not able to infiltrate that more?" Sen. Joe Manchin, D-W.Va., asked Cyber Command leader Adm. Mike Rogers during a hearing on the agency's budget for fiscal 2017.

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"The idea that you're just going to shut down the Internet given its construction and complexity is not realistic," Rogers responded. "It's just not that simple. I wish I could say that there's a part of the Internet that was only used by a specific set of users."

Manchin maintained it was a common question from his constituents. "I've had people ask me, can't you just stop it from that area of the world where all the problems are coming, be it Syria or in parts of Iraq or Iran," he said.

"I'm not just trying to find an answer, because that question is asked like shut her down, like you do your telephone, but it doesn't work that way," Manchin concluded.

The comments came in the context of the broader cybersecurity threat potentially posed by the Islamic State. "They've harnessed the power of the information arena to promulgate their ideology on a global basis, to recruit on a global basis, to generate revenue and to move money as well as coordinate some level of activity on a large dispersed basis," Rogers told the panel.

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"What concerns me when I look at the future is, what happens if a non-state actor, ISIL being one of them, starts to view cyber as a weapons system? That would really be a troubling development," Rogers said.

Presidential contenders Hillary Clinton and Donald Trump have both suggested "closing up" components of the Internet in order to combat terrorism, though those proposals have been largely met with ridicule.