Recent intelligence indicating that bomb-makers from Yemen have joined forces with terrorists in Syria to create a new strain of undetectable explosives is “more frightening than anything” Attorney General Eric Holder has seen on the job, he told ABC News.

“It’s something that gives us really extreme, extreme concern,” Holder noted, calling it ”a deadly combination” of individuals with technical expertise “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.”

Image source: ABC News via YouTube

Image source: ABC News via YouTube

Since Syrian terrorists may be plotting to down a U.S.- or European-bound plane — aided by any number of thousands of Americans and other foreign fighters possessing U.S. and European passports who’ve joined terrorist groups in the region — U.S. officials have stepped up security at overseas airports.

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

More from ABC News:

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.

Among the measures taken, DHS said that if some overseas passengers flying to the United States want to bring cell phones and other electronic devices onboard, they must demonstrate 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.