Story highlights Holder is concerned about a mix of bomb-makers and foreign fighters

The U.S. attorney general talks to ABC's "This Week"

Recent security upgrades are "not a test," he says

U.S. Attorney General Eric Holder is worried about wholes that are greater than the sum of their parts.

Take, for instance, the current mix of bomb-makers from Yemen with foreign fighters in Syria.

"That's a deadly combination, where you have people who have the technical know-how along with the people who have this kind of fervor to give their lives in support of a cause that is directed at the United States and directed at its allies. And it's something that gives us really extreme, extreme concern," he told ABC's "This Week."

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He estimated there are about 7,000 foreign fighters in Syria, coming from places like Europe and the United States.

"In some ways, it's more frightening than anything I think I've seen as attorney general," he said. "This is a situation that we can see developing and the potential that I see coming up, the negative potential I see coming out of the facts in Syria and Iraq now, are quite concerning," he said.

Since it started in 2011, the conflict in Syria has left more than 150,00 dead and some 680,000 injured.

Then there's the humanitarian crisis for the survivors, with the United Nations estimating that more than 9.3 million need help and that at least 6.5 million have been forced from their homes into other parts of Syria. Another 2.5 million are now refugees in neighboring countries.

And threats from that part of the world aren't the only ones Holder has to worry about. He also fears potential attacks on planes.

This month, the Transportation Security Administration said that security screeners at overseas airports may ask U.S.-bound passengers to turn on their electronic devices to prove they work and aren't explosive devices

It's part of an update to security measures aimed at combating potential new threats from terrorists in the Middle East and Europe.

"This is not a test ... We're doing something in reaction to things that we have detected," Holder told ABC about the upgrades.

"We are at a dangerous time," he said.