The Department of Homeland Security is increasing security measures at airports overseas amid deepening concerns that terrorists in war-ravaged Syria are trying to develop a new generation of bombs that could be smuggled onto commercial planes.

In a statement Wednesday afternoon, DHS Secretary Jeh Johnson said his department has been assessing "the global threat environment," and he has directed the Transportation Security Administration "to implement enhanced security measures in the coming days at certain overseas airports with direct flights to the United States."

Sources told ABC News that DHS was planning to issue a series of urgent directives to airport authorities and airlines operating in Europe and elsewhere, requiring them to further scrutinize U.S.-bound passengers' electronics and shoes, set up more explosives detection machines, increase random screenings of travelers, and take a series of secret actions the public would never see.

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It is believed at least some of those requirements are part of the "enhanced security measures" announced by DHS on Wednesday.

"We will work to ensure these necessary steps pose as few disruptions to travelers as possible," Johnson said in his statement. "We are sharing recent and relevant information with our foreign allies and are consulting the aviation industry."

One source said such a decision to enhance security measures overseas "is never a decision that [DHS] makes lightly."

"It's something that has to be warranted," the source said. "It has to be something particularly serious for [DHS] to even consider increasing security."

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."

Asked Monday by ABC News whether his European counterparts were doing enough to address the threat emanating from Syria, FBI Director James Comey said European authorities are "doing a tremendous amount of work" but that it was "hard for [him] to say" if it's enough.

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, the latest intelligence shows that the two threats have bonded in an unusually powerful way, essentially creating a sum more worrisome than its parts.

After coming across the initial thread of intelligence earlier this year, U.S. officials, in regular consultation with the White House, developed a "detailed plan" for addressing the issue, as one source put it. That plan was presented several weeks ago to the White House, which has been debating with other agencies whether the potential security benefits outweigh the potential impacts on partners overseas.

Getting foreign governments all on the same page is no easy task, and forcing authorities overseas to increase security measures demands more manpower and resources from those foreign governments and airlines. Nevertheless, while airlines overseas could refuse to comply with DHS security directives and drop U.S.-bound flights from their routes, economic pressures virtually eliminate that possibility.

This would not be the first time DHS has issued urgent directives to airlines overseas. In July 2006, TSA issued at least two "emergency amendments" to foreign carriers, detailing "enhanced security measures for all flights" to the United States. A month later, the U.S. and British governments disclosed that they had thwarted a plot to down several planes with liquid explosives hidden in carry-on luggage.

The latest potentially lethal partnership in Syria 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.

Since January, officials with access to the country's most sensitive intelligence have warned publicly that hard-to-detect "technologies and techniques" were being exported to Syria, that foreign fighters from the West were "learn[ing] new things" and "build[ing] new relationships" in Syria, and that "training complexes" were popping up there to prepare Western fighters for terrorist attacks against their home countries.

More recently, Comey, the FBI director, said the U.S. 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.