FBI Director James Comey said Sunday he knows of roughly a dozen Americans fighting with extremist groups in Syria, but acknowledged there could be more.

Comey made the assessment during an interview with CBS’s “60 Minutes,” his first major TV interview since he began running the agency in September 2013.

Comey said he would do everything in his power to monitor the militant fighters if they return to the United States, amid fears they would launch a terror attack on American soil. However, he said American citizens have the right to freely enter and exit the U.S.

“Ultimately, an American citizen, unless their passport is revoked, is entitled to come back,” Comey told CBS. “So someone who has fought with (Islamic State) with an American passport and wants to come back, we will track them very carefully.”

The FBI's estimate of the number of American fighters previously has been reported by Fox News and other outlets. Comey said he would describe the terrorist networks in Syria as a "metastasis of Al Qaeda," in particular Al-Nusra and the Islamic State.

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"They are both vicious, sort of the inheritors of a lot of the mantle of Al Qaeda and present different threats in a lot of ways," he said.

However, Comey said currently the U.S. is not in as dangerous a time as when Al Qaeda was at its peak, because the intelligence community is "better organized."

"We’re better organized and equipped at the border," he said. "We have relationships with our foreign partners. All of which make us better able to see dots and connect dots. The transformation since before 9/11 is striking."

Comey also said that the Khorasan Group, the terrorist organization in Syria that was targeted in U.S. military strikes last month, is a "serious" group bent on harming the U.S. The Obama administration has been under fire after it was accused of coining a new name for an old nemesis by dubbing an Al Qaeda offshoot “The Khorasan Group" to avoid invoking the name Al Qaeda.

"Khorasan was working and you know, may still be working on an effort to attack the United States or our allies, and looking to do it very, very soon," Comey said. "I can’t sit here and tell you whether it’s their plan is tomorrow or three weeks or three months from now. Given our visibility we know they’re serious people, bent on destruction. And so we have to act as if it’s coming tomorrow."

Comey also addressed cyber crime, calling it a looming threat about which most Americans are not wary. He said Chinese hackers target the intellectual property of U.S. companies in China every day.

"I liken them a bit to a drunk burglar. They're kickin' in the front door, knocking over the vase, while they're walking out with your television set. They're just prolific. Their strategy seems to be: 'We'll just be everywhere all the time. And there's no way they can stop us,'" Comey said.

The Justice Department earlier this year announced a 31-count indictment against Chinese hackers accused of breaking into computer networks at steel companies and the manufacturers of solar and nuclear technology, with the goal of gaining a competitive advantage. China has denied the allegations.

Comey said one of his top priorities is to improve the agency’s capability against such cyber crimes as ID theft and industrial espionage.

The Associated Press contributed to this report.