Latest Syria Threat 'More Frightening Than Anything' Else, Holder Says AG says evidence suggests the terrorists are one step away from a specific plot.

 -- 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.

Specifically, associates of the al Qaeda affiliate in Syria -- the Al Nusrah Front -- and radicals from other groups were teaming up with elements of the Yemen-based group Al Qaeda in the Arabian Peninsula, which built such innovative devices as the "underwear bomb" that ultimately failed to detonate in a plane over Detroit on Christmas Day 2009.

U.S. officials have been outspoken about the dangers posed by Al Qaeda in the Arabian Peninsula and -- separately -- the threat of foreign fighters in Syria, but the latest intelligence shows that the two threats have bonded in an unusually powerful way, essentially creating a sum more worrisome than its parts. And more recent intelligence has increased the concern.

The latest potentially lethal partnership is at least part of what sparked an advisory to airlines earlier this year to look out for explosives-laden toothpaste tubes, cosmetics and shoes.

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And just days before the July Fourth holiday -- coming several months after the initial intelligence was obtained -- the Department of Homeland Security announced it would be increasing security measures at certain airports overseas.

In particular, DHS said that if some overseas passengers flying to the United States want to bring cell phones and other electronic devices onboard with them, they're going to have to show that the devices can turn on.

"Powerless devices will not be permitted onboard the aircraft," the Transportation Security Administration, part of DHS, said in a statement.

It's unclear exactly which airports overseas will have to implement the expanded security measures. TSA has been in close contact with all carriers affected by the new measures, particularly American, Delta and United airlines.

In his interview with ABC News for "This Week with George Stephanopoulos," Holder called the new security measures "appropriate" and "prudent steps that are necessary to protect the flying public" in light of developments in Syria and neighboring Iraq, where the group Islamic State of Iraq and Syria -- or ISIS -- is now wreaking havoc and recruiting Westerners to fight.

"This is not a test," Holder said of the new measures. "We're doing something in reaction to things that we have detected."

Holder insisted he's "really confident in our capacity to understand what's going on, to monitor what's going on and working with our allies."

FBI Director James Comey recently said the government is spending "a tremendous amount of time and effort trying to identify" anyone who's gone to Syria, but "the challenge" is not missing anyone.

About 7,000 foreign fighters, including dozens of Americans, have now joined nearly 16,000 other fighters operating in Syria.

While in Norway, Holder publicly addressed European officials and advocated for a multi-national strategy to stem the flow of foreign fighters, urging other countries to conduct more undercover investigations and enact stronger laws criminalizing support for terrorists.

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