The Obama administration may ask overseas partners to enhance security measures at airports and is weighing whether to do the same here at home to address deepening concerns that terrorists in war-ravaged Syria are trying to develop a new generation of bombs that could be smuggled onto commercial planes, ABC News has learned.
"[This threat] is different and more disturbing than past aviation plots," one source said.
The issue was discussed this past week at the White House during a meeting of top-level officials from intelligence agencies, sources said.
For months the Department of Homeland Security, FBI and other agencies have been quietly debating whether to boost the U.S. security posture and encourage overseas partners to take action too. The agencies have also been debating whether to make a public announcement on potential new security measures at airports.
The back-and-forth has been based on intelligence showing 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, U.S. officials learned that 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.
Intelligence obtained by the U.S. government, however, has not indicated a specific target or a specific timeline.
While 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, began developing plans and potential security measures to address the threat. The development of those plans is now in its final stages, according to sources.
It's unclear exactly what new measures are being considered for U.S. airports and U.S. stations in airports overseas. But one source said new measures could include increasing the rates of random screenings at airports, targeting certain types of travelers, or more obvious changes for travelers going through security lines.
The 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.
Rep. Peter King, a top Republican on the House Homeland Security Committee, said security at airports overseas is "a real concern" to officials in the U.S. and that the federal government needs to be "very aggressive" is responding to it.
"I can’t go into all the details but that is very important to do because a number of airports do not have the type of security that they should have," King said Sunday on ABC News' "This Week," speaking about boosting measures at airports overseas.
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.
In May, FBI Director James Comey told a Senate panel the U.S. government has a plan to mitigate the direct threat to the U.S. homeland from Syria, though he declined to offer any details.
More recently, Comey 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.
In an exclusive interview on Thursday with ABC News' George Stephanopoulos, President Obama noted that "battle hardened" foreign fighters in Syria are increasingly slipping over porous borders and joining terrorist groups in Iraq, where the group Islamic State of Iraq and Syria -- or ISIS -- is now wreaking havoc.
Though al Qaeda denounced and cut ties with ISIS earlier this year, elements of both groups are committed to attacks against the West.
"Then they come back, they've got European passports ... [and] don't need a visa to get into the United States," the president said of certain foreign fighters.
As part of a much larger request to fund U.S. military operations, the White House has asked Congress to approve $500 million so that U.S. forces can train and arm "moderate" rebels fighting against the Syrian regime.