Latest Syria Threat 'More Frightening Than Anything' Else, Holder Says

— -- Recent intelligence indicating that bomb-makers from Yemen have teamed up with terrorists in Syria to develop a new generation of undetectable explosives is "more frightening than anything" else the Obama administration has seen, Attorney General Eric Holder warned, becoming the first U.S. official to publicly confirm ABC News reporting on the threat.

In recent days, U.S. officials have boosted security measures at airports overseas amid deepening concerns that terrorists in war-ravaged Syria could be looking to down a U.S.- or European-bound plane, with help from one of the thousands of Americans and other foreign fighters carrying U.S. and European passports who have joined terrorist groups in the region.

Holder called it "a deadly combination," in which people with technical know-how are now "married" to "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."

"It's something that gives us really extreme, extreme concern," Holder said. "In some ways, it's more frightening than anything I think I've seen as attorney general."

Holder sat down for a rare and exclusive interview with ABC News in London, where the attorney general was wrapping up meetings with U.K. officials and other European counterparts to discuss the issue.

Days earlier, Holder held similar meetings with officials in Oslo, Norway, to "exchange information in the way that you can't if you're not present" and "come up with techniques to thwart those who would try to do us all harm," as he described it.

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Earlier this year, U.S. officials learned that a particularly extreme "subset" of terrorist groups in Syria was working alongside operatives from al Qaeda's prolific offshoot in Yemen to produce "creative" new designs for bombs, as one source put it.