Cell Phones to Be Scrutinized by TSA Expanded security measures to be implemented overseas.

WASHINGTON -- If some overseas passengers flying to the United States want to bring cell phones and other electronics onboard with them, they're going to have to show that the devices can turn on, the Transportation Security Administration said today.

The move comes just days after the Department of Homeland Security announced TSA would be increasing security measures at certain airports overseas amid what ABC News learned was deepening concern that terrorists in war-ravaged Syria are trying to develop a new generation of bombs that could be smuggled onto commercial planes.

"As the traveling public knows, all electronic devices are screened by security officers. During the security examination, officers may also ask that owners power up some devices, including cell phones," TSA said in a statement. "Powerless devices will not be permitted onboard the aircraft."

TSA said travelers themselves "may also undergo additional screening."

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.

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DHS is also directing airport authorities and airlines operating in Europe and elsewhere to further scrutinize U.S.-bound passengers' shoes and increase random screenings of travelers, sources told ABC News.

"We will work to ensure these necessary steps pose as few disruptions to travelers as possible," DHS Secretary Jeh Johnson said in a statement Thursday.

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.

Bolstered by more recent intelligence, U.S. analysts believe the "subset" of extreme terrorists in 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 Al Nusrah Front and other groups in the region.

While intelligence obtained by the U.S. government has not indicated a specific target or a specific timeline, one source called the threat "different and more disturbing than past aviation plots."

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.

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.

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.

On Sunday, TSA said it "will continue to adjust security measures to ensure that travelers are guaranteed the highest levels of aviation security conducted as conveniently as possible."

ABC News' Matt Hosford contributed to this report.